# Presentation: 801762

## Electronic properties in nc-Si:H

- Vertical Mobility in device type structures, both electrons and holes, in hot wire and VHF materials
- Channel mobility of holes in TFT devices
- Diffusion lengths and influence of built-in fields
- Hole lifetimes
- Capture cross-sections of recombination centers

## Measurement of vertical mobility in device type structure

- Use space charge limited currents
- n+nn+ on stainless steel
- Only change from devices is replacing top p+ with n+
- Gives mobility in the same transport direction as in devices
- Can also do it for holes p+pp+ structures-not much work so far

## SCLC

- J = 1.12  V2/L3
- V is voltage and L is length of n layer
- Good ohmic contacts necessary and extreme care in reducing all stray resistances– sample resistance are LOW, few ohms
- Use highly doped amorphous or nanocrystalline Si n+ layers

## SCLC

- Must not get trap controlled SCLC
- Necessary that ohmic (pre-sclc) Fermi level be above the traps, which are ~0.35 eV below Ec
- That means (Ec-Ef)<0.35 eV in n type
- Typical conditions in our samples, with doping of few 1014 to 1015/cm3.

## SCLC n+nn+-follows the expected curves

## Mobility of electrons in nc-Si

**Caveat: These mobilities will be lower**
**Than actual, because of nonuniformity **
**Of crystallization with thickness**
**A-Si near substrate will soak up voltage**

## Hole mobility in p+pp+ samples

**Hole mobility in p+pp+ samples**

**Mobility of ~2.5 cm****2****/V-s**

## Measurement of diffusion length

- Thin P+layer

**Photons**

- Depletion
- Width Wd

- n layer

**L****d**

**Change voltage, change W****d**
**QE~******** (W****d**** + L****d****) for small ******

## Relative QE vs depletion width for varying defect density materials

**Scale is 0.1 micrometer**

## Ld2  vs. 1/defect density

**Shockley-Read-Hall model applies**

## How does one improve dif. lengths?

- By improving material
- Or by improving internal electric field-grading of dopant or bandgap

## ppm levels of B Dopant grading

- Increase B

- E**F**

## Influence of B grading on dif length-QE analysis 

**Very little change**
**In QE with **
**voltage**

## Very large apparent diffusion lengths >> sample thickness

**Scale is 1 micrometer**

**B grading works to **
**increase hole collection**

## Measurement of lifetime- Reverse recovery in p+nn+ junctions

- Inject holes from p+ into n
- Reverse bias and study current
- There should be a relatively constant current regime ( storage time) and a decay
- Storage time Ts related to lifetime 
-    Ts =  [ erf-1 { If/ (If + Ir)}2]

## Reverse recovery transient

- Voltage pulse

- Current pulse

- Storage time

- Time

- Time

## Calibration:Transient on a c-Si commercial diode with short lifetime

**100 ns**

## Measurement of carrier lifetime-nc-Si

## Plot of storage time vs. error function

## Lifetime vs 1/defect density

**Slope gives capture cross-section ~ 1-2 E-16 cm****2**
**Not a metallic impurity**** **

## Hole mobility in p+nn+ devices

- Simultaneous measurement of lifetime, diffusion length, defect density and doping in the _**same**_ device
- Lifetime and defect density yield capture cross section of recombination centers
- Lifetime and dif. length yield hole mobility

## Hole mobility deduced from Dif length and lifetime

**Mobility data very similar to ones in p channel TFT devices-see poster**

## Hole mobility from p channel TFT-VHF nc Si

**Transport**
**channel**

## TFT I-V curves, p channel/n body device

## Hole mobility in nc-Si from Sqrt (Ids) vs. Vg curve

**Mobility=1.3 cm2/V-s**

## Conclusions

- All the measurements indicate crystalline type behavior with much smaller carrier mobilities – diffusion controlled device
- Graded doping increases effective diffusion length significantly- indicates that ppm doping does not destroy the material
- Both electron and hole mobilities measured using variety of techniques, both in vertical and horizontal (channel) direction
- Mobility values are a few cm2/V-s