III-V Semiconductors - Science topic

Any hints regarding other porous III-V semiconductors are also welcomed.

Does exist any interaction between the silicon contained into a silicon doped semicondutor and the acetone used to clean the sample? I use the sonicator.

Hello,

I want to model an heterostructure device with the drift-diffusion model, for that I need a boundary conditions. At the heterostructure there is a junction barrier that dictates a thermionic emmition current injection boundary at the junction (J=A*T^2*exp(Phi-dPhi/vt)). Moreover, due to poor surface conditions there is a high surface recombination, which is modeled as Jn=qS(n(0)-np0).

*Phi and dPhi are the junction barrier and reduced junction barrier respectivally.

Does these boundary conditions (thermionic emmition and surface recombination) can be combiend together or they equivalent in some degree?

Not sure how to think about this situation.

Thanks.

If we have a diode with p-type (Germanium) and n-type ( Silicon), then what will be the formula to find the built in potential?

Can we learn things from the shape of the IV curve of a diode in order to examine issues and phenomena that exceeds from ideal behavior? Things like the slope of the reverse or forward bias, the knee point s-shape, etc.

What mechanisms, for example, tunneling, SRH, strain in heterojunctions, shunt paths, schottky contacts, too high series resistance, hot carrier effects, panch-throghs, traps, non ideal doping, etc. These are known effects but I'm not sure if and how they reflect in the IV curve or if there are other less known effects that I missing out.

Are there good reviews summarizing different issues and effects in the device (design, process, fabrication) and how they con reflect on the IV curve?

Most material I found talks about these effects but in the context of desired effects on different devices.

Where can I start the learning of issues in PN diodes, or semiconductor structures in general.

Any input on this matter will be highly appreciated.

Thanks

What will be the future of solar cells? Perovskite solar cells? OR III-V Solar cells ??

1. Perovskite solar : Easier to fabricate (can be printed like paper) and have low conversion efficiency but might include toxic materials.

2. III-V solar cell: Difficult to fabricate, however, it can provide higher efficiency (47.1%, 2020).

Group-IV elements doping in III-V behaves amphoteric: n-type if the dopants occupy the group-III sites and p-type if they occupy the group-V sites, which is not hard to understand.

My question is on a group-VI elements doping in III-V semiconductors. An example is Te doping in GaAs. Te atoms are known to occupy As sites and make GaAs n-type. When we excessively dope Te in GaAs, however, we observed that GaAs became p-type. I would guess that it happens as Te starts occupying Ga sites or interstitial sites.

First, has any of you observed this (p-type conductance from group-VI doping in III-V)?

Second, can anyone help me understand the mechanism?

Thanks in advance!

I was trying to understand Lattice deformation and stress in epitaxial layers, when comes to the inclination angle △φ (lots of articles just give the equation...), it confused me a lot. I totaly have no idea how to get it.

Hello, everyone,

I'm a little confused about valence band structure of Aluminum nitride (AlN). It is known that the crystal-field split energy in AlN is negative, so there are a lot of papers saying that the crystal-field split-off hole band is the topmost.

On the other hand, according to the k.p method (for example, or ) valence band energies at k=0 can be expressed as shown in the picture below (where Δ₁ is the crystal-field splitting).

For Δ₁>0 (like in GaN or InN), E₁>E₂>E₃ and the bands are usually reffered as HH, LH and CH, respectively, so the topmost band is the heavy-hole band.

For any values of Δ₁ and Δ_2, E₂ is always larger than E₃, so in the case of AlN E₂>E₁>E₃ which should mean that the topmost valence band is the light-hole band. Am I missing something obvious?

Another quick question is about an appropriate interpolation scheme for hole effective masses of ternary alloys. Should we use linearly interpolated Luttinger-like parameters A₁...A₄ for a certain composition to calculate the effective masses (including strain effect if needed) or should we calculate effective masses for the binaries from their parameters A₁...A₄ first and then interpolate between the obtained effective masses?

I am doing some FIB cuts and analyzing my layer stack. I always see dots in the InP and InGaAsP layers, but I don't know their origin. These dots do not appear when I do a SEM of a cleaved sample, so I assume the origin is related with the Ion beam process. I attached a picture of a SEM image after an FIB to make clear which dots I am referring to. Any help or reference to solve this question is highly appreciated!

The punch through mechanism is described as reverse bias applied to drain, which results into extended depletion region. The two depletion regions of drain and source therefore are intersectioned with each other, and this results into "one" depletion region, and flow of leakage current and consequently breakdown of MOSFET.

My question (by looking at the attached figure) is that how intersection of two depletion regions with electric fields in opposite directions (Eleft and Eright) can results in "one" depletion region. I believe opposite electric field cancel each other out and create a flat band region wherever the two depletion region coincide.

Can anyone please add some explanation to this phenomena?

More explanation on punch through:

Cheers,

Amir

I am ICP etching AlGaAs Bragg structures using Ar, Cl2 and BCl3.

After etching (typically a few microns), there is material deposited on the surface and sidewalls of my photonic structures. I assume it is something like AlCl3... but I'm not sure.

I want to remove this deposited material as it increases the roughness.

I have tried H2O2, up to ~20% concentration, but this does not seem to help.

Does anyone know what the deposited material is, and how could I dissolve it away?

I made a cut 60 degree to the primary flat {0-11) and made a sample of 4X8 mm with primary flat as the base. The shape of the sample is a parallelogram and I made scratch parallel to the primary flat (for the initiation of cleave), clamped it in a special sample holder for X-STM analysis and then tried to cleave it inside the STM chamber in UHV.

The cleave isn't atomically flat (110) but it has a lot of step edges as shown in the figure. I tried to cleave around 15 samples. Most of them look exactly the same way.

Any suggestions or ideas to cleave it properly are welcome.

Hi there,

I'm looking to buy various photodiodes/solar cells for some experiments.

The cells I'm looking for are InGaP, CZAS (and CZT) and amorphous silicon.

Any pointers to where I could buy those type of cells? The higher the efficiency of the cells, the better but not the most important factor.

Thank you in advance.

J

I know that Rashba coupling parameter magnitude in experiments is about 0.1 eV.A. I also have seen some papers that has reported gigantic Rashba couplings about 0.3 eV.A or higher magnitudes. If fact, I need to know that can we have small quantum rings (radius 20-60 nm) with about 0.2 eV.A or higher Rashba coupling?

Hi everybody, 

I am preparing the presentation for my phd thesis and I would like to add some really concrete motivations to the introduction slides. My thesis focuses on the 3D-growth of III-V semiconductors monolithically grown on silicon. In the first part of my talk I should introduce the defects due to the monolithic growth of III-V on silicon and the different strategies used to filter these defects.

Therefore, I was thinking of starting with a slide of main motivation which will let the audience understand the following problem: defects affect the cost in semiconductors devices, therefore manufacturing companies are wasting money due to this. If we solve this problem we could potentially save this amount of money. However, I want to be more clear than that and give them some actual numbers. How much do defects cost for manufacturing companies ?

Of course, it would be amazing if you could point out some market analysis (qualitative and quantitative analysis), survey papers, etc. Thanks for your help. Ida LUCCI

I know that in order to alter the non-linear properties of materials like glasses, poling is used. My question is, is there any way to achieve that in GaAs? Thank you!

We consider purchasing a new notching/scribing tool. The major intended applications using such piece of equipment are as follows:

• Cleaving GaAs/InP/GaSb/InAs laser diodes for smooth facets

• Dicing laser bars

• Photonic Integrated Circuits (PICs) singulation

• Limited-Production Dicing

I wanted to check if any of you have any recommendations for a particular piece of equipment from any vendors (except Loomis), which can do these tasks very reliably. I prefer an automated system rather than a manual one.

I am trying to measure the I-V characteristic of a Schottky diode (ZnO on ITO) using a standard IV measurement machine. The sample was illuminated using SoLux lamp. The value of FF that it gives, is negative...It definitely does not sound right (considering FF=Vmp*Imp/Voc*Isc). Also, why the efficiency is infinity?

Does anyone have an explanation?

P.S: Results as shown below:

-8490155.3 Fill Factor [%]

2.947E+2 Pmax [mW]

-1.593E+0 Vmax [V]

-1.850E+2 Imax [mA]

-9.59E+0 LIV Rshunt [ohm]

-9.58E+0 Voc slope [ohm]

Inf Efficiency [%]

TIA

Hello,

I have been working on Au/n-GaAs (Si doped) Schottky junctions. It is very well know that dark reverse saturation current (J0) is primarily dependent on temperature and nothing else (in ideal case I assume!).

However, I can clearly see a doping concentration dependence on my experimental results, which I cannot figure out. It is observed that J0 is usually 100-1000 times smaller for doping concentration of 10^16 cm-3 than 10^17 cm-3.

Can anyone explain this?

Thanks,

Amir

Hi

Currently I am working on growth of epitaxial ternary and Quaternary III-V semiconductor alloys.

For my research it is needed to know the exact alloy composition using RBS.

So, Please help.

Thanks in advance.

I want to simulate a III-V solar cell for various temperature starting from 0 to 300 degree Celsius. Please any one may help me out by providing brief idea about it. 

Hi

i am am trying to etch down NI layer with thickness of 100nm on nGaAs substrate. I have read reference articles mainly the handbook of metal enchants and tried using following solutions:

H2SO4:H2O ..... different ratios from 3:1 to 4:3 to 1:50. However the Ni layer did not etch but the substrate started to be etched which is not desired.

HNO3:H2O 3:7 .... not etching Ni

HNO3:HCL .... etching both nGaAs and Ni

Afte observing that the Ni is not being etched, I believe there maybe NiO on top of Ni that needs to be etched away. However to remove NiO, there is need for HNO3 which is not very appropriate for using with nGaAs. Any recommendations for etching the Ni layer?

Thanks

Can the concept of the N-face or the Ga-face exist when the GaN film is not a single crystal anymore but a polycrystalline film? Can there be something like a net polarity in poly films?

Hello,

Could someone please point me to some reference data on the pemittivity of semi insulating GaAs in the THz region (1-4 THz). Chochol et. al have published some work on doped GaAs but I'm looking for semi insulating GaAs.

Thanks.

Chochol work here:

Hi,

I have deposited 100nm Au on 5nm Au using e-beam evaporation and conventional photo-lithography techniques. I used ~5nm Ni as an adhesion layer for 100nm Au, so basically the structure is Au 5nm/Ni 5nm/Au 100nm.

The lift-off of the 100nm Au is extremely hard. I usually rest the sample in acetone for couple of hours so the lift-off is complete. However, in this case even after days, the lift-off is not complete and there are lots of unwanted pieces of Au on the surface. I try to use ultrasound (40kHz) as little as possible because it damages to the structures and removes the main metal layer as well. However, for these samples I apply ~10sec ultrasound, but after ultrasound the quality of the 100nm Au is bad and so much of metal is gone.

Worth mentioning that I dip the samples in HCL (37%) for ~3sec and H2O for 5sec before the 100nm Au deposition to remove any unwanted organic particles on the surface.

Is there any solution to improve the deposition/lift-off process, so the final 100nm Au has a good quality and shape?

Thanks for your time!

Amir

I have fabricated a Schottky junction (100nm Au on nGaAs substrate with Ohmic contact of AuGe(100nm)/Ni(30nm)/Au(50nm) on nGaAs) using ebeam evaporation...When I perform I-V measurements, there has been times where a current range of 500mA (or sometimes even lower) passes through the device and after that there is black hole (dot) on the surface of Schottky layer (where probe was touching). The device I-V characteristics becomes linear (previously it was a nice diode-like curve) and to me it seems like device is failing. What can be done to increase the tolerance of the device to withstand higher currents?

Silicon nitride films (SiNx) can have a broad range of x-values (i.e., Si rich or N rich). Can similar be expected for GaNx?

Dear Experts,

Suggest me to mix two type of catalysts with one another, which in solid state form. I am also excepting with uniform perception.

Hello everyone,

I have used Silicon and Magnesium as dopant materials for different layers of GaN photodiode. I have drawn the structure and used those dopant materials using Sentaurus TCAD. I was able to do meshing. I visualized doping profile in Meshing section.

After running simulation, I had the following issues:

Net doping concentration (DopingConcentration): not found in doping file

Total doping concentration (TotalConcentration): not found in doping file

No Species in doping file for incomplete ionization. !

Can someone please guide me how I can resolve the issue. I have not faced this kind of problem in Sentaurus TCAD while I was working on different types of Silicon photodiode devices with Boron and Phosphorus doping.

Thanks,

Mottaleb

In a research paper lattice parameters of GaN is

a=0.3232nm and c= 0.5269nm   c/a =1.630   B.G=1.596 eV

for InN

a=0.3628 nm and c=0.5870nm  c/a=1.618    B.G=0.001eV

But it is reported that lattice mismatch of GaN and InN is 10.0% for a and 9.3% for c.

how to find these lattice mismatch?

have any specific formula?

I am trying to use a small size crucible (smaller in volume) in ebeam evaporator for depositing expensive materials such as Au and AuGe. I also want the crucible to fit the pocket of the evaporator. Attached you can see the original size crucible that is being used and below that you can my design of smaller crucible. Smaller crucible is made by thickening the walls of the original size crucible (crucible material can be graphite copper or tungsten). However, the issue here is that the thick walls of the crucible may degrade the cooling process and result in crucible to break (crack). 

What is the possible solution for having a smaller size crucible here?

I heard from a friend of using something like a "reducer" that I can fit inside the original size crucible and deposit my material. This reducer can be made of copper for example, which has good heat transfer. 

Dear all,

I am not chemist and I can't really find the answer to my question in the litterature.

Could anybody suggest me a recipe for a non-selective wet etching of InGaAs and InP with similar etch rates? If possible I would like to avoid Brome based mixtures.

I was wondering if mixing in the same solution mixtures of H2SO4:H2O2:H2O (etching InGaAs) and HCl:H2O (etching InP) would do the trick. Any chemist could confirm?

Thanks a lot for your help.

Thibault

Hi everyone,

I want to dry etch III-N materials using BCl₃ in a ICP-RIE system and I wonder if there are risks of samples or system contamination using this gas ?

I have read a fair few papers about this: defect models, papers that deal with demonstration of Fermi level depinning in Ge using certain top metal or MIS combinations. There is no doubt that the pinning position is near the valence band (VB) as I have confirmed this by experimentally measuring Schottky barriers for various metal/ Ge combinations (on both n and p type Ge).

Fermi level pinning is usually attributed to a distribution of donor and acceptor states near the VB forming a charge neutral level (~0.1 eV above the VB). 

Unlike GaAs, I could not find any intuitive explanation for the physical origin of these defects (antisite defects seem to be the main candidates that behave like donor and acceptor states in III-V's). Moreover, segregation of group V atoms at atomic terraces are known to create dipoles that could affect the surface workfunction and local charge. Together, these phenomenon may somehow account for fermi level pinning. However, in Ge, there's only one species (Ge atoms) so I'm finding it hard to understand how both types of defects may arise. Are point defects (interstitials and vacancies) alone sufficient to create this effect? Could you provide any references that may be helpful? Thanks.

Why depletion region (i.e. built-in potential) in Schottky contact is smaller than the pn junction? In other words, why Voc of Schottky contact is smaller than the pn junction?

Does it have any relation with the types of charge carriers in these junction. I mean, since only electrons are the charge carriers in Schottky contact, therefore, the built-in potential is lower than the one in pn junction, where both electrons and holes are charge carriers and create stronger depletion region?

In a Schottky solar cell, consider using an opaque metal on top of the bulk of semiconductor.

Depletion region exists mostly between the metal and semiconductor, and due to the opaque metal, therefore, solar illumination cannot  reach the depletion region. Therefore, no electron and hole are created in this region. However, electron and hole can be created in other areas in the bulk of semiconductor (outside of depletion region). Can these electron have any effect in creation of photogenerated current?

I want to use RIE to etch GaN, I think I should use Cl2 and BCl3 but I need to know about the range of plasma power and working pressure.

I would use them as the semiconducting llayer in transistor devices and as we dont produce them in our lab I need somewhere where i can purchase them.

Dear Experts.,

I got Cu doped SnO₂ Nanoparticles with size (<25nm). Already i got good results in XPS compare to pure SnO_2. In magnetisation also i got good ferromagnetic signals at variable temperatures (300 to 5 K). But i need to confirmed it mine is Dilute Magnetic Semiconductors or not ? 

Pleased to give some supportive information with evidence.

Increment of Metal loaded in semiconductor shows increasing bandgap with various composition.

Normally, Bi are considered to replace V spices in III-V-Bi alloys.( For example, the most widely studied GaAs_1-xBi_x). However, it might be not  the case in InPBi.  It is a crucial problem for our future studies.

Recently, we grow InPBi epitaxial  layers on InP(100) surface by gas source MBE and many novel  phenomena are observed. For example, the mid-infrared photoluminescence and new raman peaks at 150 and 170 cm^-1  which remind us the Bi_n clusters (the new raman feature may also be expected when InBi bonds are formed. From the STM results, up to now, we were just able to see the P atoms of (110)  face where some of them are replace by Bi atoms ). I hope there are other methods to solve this problem. Welcome suggestions!

Dry etching methods for III-V photonic crystal slab layers are needed to complete our project.  

If you have ICP-RIE for III-V etching, please contact me and we can have a discussion.

Thanks very much! 

I am wondering if i have a GaAs solar cell and a Si solar cell, what would be the best combination to achieve the max power:

have a two modules  of series connected silicon (gaas) cells together, and then connect it to other module of series connected gaas (si) cell

or 

connect one si cell to a gaas cell and then  make a module out of this combination?

I start work in the modeling material science. I doped GaN with RE and found that the impurities energy in the middle of Conduction band and valence band. 

How to calculate the Band gap energy ? are we calculate like donor or acceptor ? 

blue is conduction band and red is valence band, between that (the two line ) is maximum and minimum energy of impurities.

Hi,

Which geometry of the metal contact on the semiconductor wafer can give best Schottky diode properties? Whether the circular metal dots or the large area of metal (say 1x1 cm2), which is deposited on the front surface of the wafer?

How does the performance depend on diode area?

Attached is just a rough sketch, what would be the best design?

As i just only understand that pinch off actually occur when apply some voltage on drain (VD) until equal to the drain voltage saturation (VDS) so that the depletion region at drain is increasing due to electrical field at the drain edge is decrease, thus only small electron and even no electron is induced at the drain edge. i just want to know Why actually it occur?

Hi

I was wondering how carrier scattering varies with the defects in thin film semiconductors. For example if iii-V semiconductor grows on silicon substrate because of the lattice mismatch there will be lot of misfit dislocations and growth twins. Is there any straight forward answer where it can be said that the carrier scattering increases/decreases with line defects or stacking faults. Or if I pose the question like this: how the mobility  is affected by these defects. Will it increase or decrease by these defects? Or is it complicated to say? There exist lot of literature which mostly are misleading or I did not find very good answer in literature which can straight way gun down the question. Some literature says carrier scattering will be more with twin length some says it would decrease with coherent twin boundary length as twin boundaries are low scattering centers. I am not sure how to interpret. In my case I got twin formation between film and the substrate which drastically increases the conductivity. I am not sure whether to interpret that twin boundaries played a dominant role to reduce the carrier scattering and thus enhanced the mobility and conductivity or it has no effect on carrier mobility or scattering.  Your thoughts will be highly appreciable.

I want to reflow microposit S1813 photoresist around some 700-1000 um tall structures, so that the tips of these have almost no resist left on them. Any tips on how to do this?

I need reference spectra to compare with. I have measured the Aluminium-K absorption edge for an AlInP layer. If anyone knows literature data or has performed similar experiments please let me know.

Thanks

Why we cannot crystallize III-V simiconductors out of solution (aqueous or organic solvent)? for example cannot obtain in VLS (vapor-liquid-solid) method and force to use SLS (solution-liquid-solid) method?

We know that most of the 2DEG-electrons in the undoped AlGaN/GaN heterostructure comes from the surface donor states (UK Mishra et. al and Guang et. al.). These electrons are driven by the electric field due to two sheet charge at the AlGaN-surface (-ve) and the interface (+ve). Now, when the electrons from the surface donor states travel to from 2DEG, they have to go past the +ve sheet charge in the interface (AlGaN-side). Why don't the electrons recombine with the +ve sheet charge before getting confined into the GaN side of the heterostructure and forming 2DEG?  

Usually III-V semiconductors have direct band gap and can be calculated directly by some mathematical manipulation, however few inorganic materials usually don't exhibit a prominent absorption band edge!!!.

How the band gape is estimated for such type of materials?

III-V semiconductors have tetrahedral crystal structure with sp3 hybridization. Does all materials that have this type of crystal structure have semiconductor properties? Does this include organic compounds also?

But other iii-v semiconductor do not have that much high band gap? What is the speciallity of GaN crystal which makes its band gap that high than any other semiconductor?Also how it consumes less power and the brightness/watt is more than any other light sources such as CFL which makes its use in LED possible?

This is a vertical Tunnel FET fabrication

I need to fabricate In_0.53Ga_0.47As  and In_0.7Ga_0.3As layers for a vertical TFET

Which case is shows more doped Metal (Cobalt) into semiconductor nano-TiO_2. 

1. Metal (0.02g) doped into semiconductor (0.1g).

2. Metal (0.02g) doped intoi semiconductor (0.3g)

What happens when we increase semiconductor ?

For coating the ceramic AlN substrate with Au using e-beam evaporator,  what metal layer should be used at first as an adhesion-promoter? Is it Ti? In fact, I evaporated Ti/Au(20 nm/800 nm) on ceramic substrates, however, I experienced the metal peeling-off problem as shown in the attached file.

Could this problem be due to the fact that the substrate is not cleaned enough prior to loading it into the chamber? Is the pre-cleaning process very critical? How to clean the substrate before loading into the chamber?

I have simulated a p-i-n photodiode in Si, which has a large intrinsic region width, to capture a broad range of wavelengths from 400 nm to 900 nm. I see that the spectral response peaks at about 625 nm. I am interested to know why this happens at this wavelength?

I would expect the curve to decrease from the lower to the higher wavelengths, due to decreasing absorption coefficient values. Or is there something that I am missing here? Thanks for any clue!

In multi-junction PV cell assemblies, why is it common for the substrate to be a metallic-ceramic-metallic structure?

What purpose does each layer serve exactly?

I know the difference between GaAs(111)A and (111)B is the surface termination. Now I have GaAs(111)A wafers for epitaxy puropse, and since it is double-side polished, can I just use the backside as (111)B, or it is not epi-ready in terms of growth purpose? On the other hand, I found little suppliers for GaAs(111)B with ONE side polished; and two-side polished wafers just increase the complexity of my experiment. Is there a special reason about this? 

Is there any relation between traps energy level and electron capture cross-section in semiconductor materials? Where can I find the theory or experimental data related electron capture cross section for the trap assisted recombination for GaAs material? Any good book suggestion will be very useful.

   

We have super high resolution (76um Pixel, <2 Arc Sec) XRD rocking curve data for a 0.5um MBE Super Lattice (SL) epi of InAs/InAsSb on (001) GaSb 25nm period. We would like to map the estimated relative epitaxial film thickness from this data.

Raw real time data showing up to 4 harmonics from SL structure, (004) symmetric reflection XRD rocking curve analysis: https://www.youtube.com/watch?v=De_Nh7iN6y4&list=PL7032E2DAF1F3941F&index=29

Comparison of the substrate GaSb (004) with Theoretical and Experimental GaAs (004): https://www.flickr.com/photos/85210325@N04/10647827636/in/set-72157648319384526   Ideally, we should have compared with the data from the substrate GaSb prior to epi deposition, in situ perhaps!

Q1. Is it reasonable to assume that the diffracting volume across the X-ray topograph is constant for an "optically flat" sample using the parallel beam geometry/optics. As long as the collimated incident beam's wavelength (energy) dispersion is constant (unchanging), this would work. Even if the beam monochromation wasn't perfect (a delta function). Correct?

A1. Seems to be "YES" based on feed-back thus far 11-11-14.

Q2. As in this case, if the estimated penetration depth is in the order of 12-15um, the epitaxial film would then diminish the substrate reflection intensity based on its thickness. Therefore, the integrated intensity from the substrate below the epitaxial film would be inversely proportional to the epitaxial film thickness. However, the sensitivity of this signal (from substrate plus 0th order peak of epi) to the epi thickness is a lot lower than the direct signal from the epi alone as we can considered in the +2nd Order Reflection (960 Arc Sec on RHS of the GaSb substrate peak in figure below of the relative reciprocal space map, Y-Omega, for the SL sample). https://www.flickr.com/photos/85210325@N04/15527541937/

A2. "Jury is still out".

The key to precision in this case would be the correct identification and deconvolution of the "background signal" and the incident beam shape factor. Using an advanced 2D detector & image processing techniques, the SNR with a <2kW conventional laboratory source doesn't seem to be a challenge. Could always use more signal :-)

Appreciate the erudite and expert RG membership's help!

I am working on two types of MSM photodetector ,on of them ohmic the other one schottcky. I studied sensitivity of both detectors, I saw sensitivity decreasing at high voltage in range (5-10)v

The question is why did sensitivity decrease at high voltage(5-10)v of MSM photodetector?

sensitivity =photocurrent/dark current

I am working on GaN. I want to determine the theoretical energy band diagram of the GaN and its hetero structures. I am very new in this type of work. Can some one help me?  

Thanks in advance.

In Schottky contact formation, surfact states/interface states play a critical role. According to the surface states nature, Schottky barrier height become indepdent from metal work function. Literacture shows, in the case of fermi level pinning, Schottky barrier height become equal to bare surface barrier height.