# Presentation: 802198

## Ta-filament HWCVD a-SiGe:H Update

**Yueqin Xu, Howard Branz, Bob Reedy and Harv Mahan**
**NREL **
**Jim Doyle (sabbatical at NREL)**
**Macalester College**
**Baojie Yang, Jeff Yang**
**United Solar **
**Shouvik Datta, Dave Cohen **
**U. of Oregon**
**Technical Help - Mark Landry, Matt Page**
**NREL**

## Ta-filament HWCVD a-SiGe:H

**We can get very good a-SiGe:H properties**

- 43-46 meV:
- Narrowest Eo’s _ever_
- _measured_ for a-SiGe:H

- Hole collection approaching
- that of PECVD a-Si:H

- U of Oregon

## Outline

-   Oxygen contamination
-   Gas-phase chemistry (depletion measurements)
-   Collaborative devices with United Solar

## It seemed we only got great films when O was low

-   Identical deposition conditions, but different O ‘levels’ (SIMS)

-   So we decided to find the source of O

- 1019-1020 of O

- 1018 of O

## Mass spectrometer plumbing

## Bursts of O2 and N2 appear ~5 min after GeH4 flows

** ****Mass 32 (O****2****) and 28 (N****2****) appear together ---> air leak **
** ****Time of burst ~ emptying time of GeH****4**** gas line**
** ****Magnitude of ‘burst’ depends on gas flow ‘history’**
**  ****after 3 day wait, burst magnitude generally bigger**
**  ****occasionally burst  disappears  **
** ****GeH****4**** line thoroughly leak checked**
** ****Implicates air contamination (bubble?) in tank**

**No SiH****4**** expt**
**because it would obscure**
**O2 peak**

- N2 - mass 28

## Solutions implemented

**(2) Install new GeH****4**** tank (Dec 05)**

**(1) New oxygen filter on GeH****4**** gas line (Oct 05)**

-   Consistent SIMS oxygen now at ~ 1018 cm-3
**   ****Let’s ruin a good thing:  Installed intentional air leak.**

##  Controlled air leak sets O-level in films

** ****We can vary O reproducibly from ~10****18**** to > 5x 10****20****/cm****3**
** ****Minimal N incorporated into films (filament doesn’t break N****2****)**
** ****Ge is constant**

- Oxygen

- Nitrogen

## Surprise, surprise!

- Oxygen reduces the Urbach slope

- S. Datta and D. Cohen

## Or was it actually the C or H level in those films?

- 1018 of C

- 1017 of C

## Gas depletion measurements

** ****- a simple monitor of deposition chemistry**

- See Tuesday poster by J. Doyle et al.

- Fin

- Fout

- to film and walls

- to filament?

-  Measure GeH4 (SiH4) partial pressure with filament on and off

-  Fractional depletion is   f = (P0-P) / P0

## Depletion measurements correlate with SIMS

- SIMS Ge fraction in film

-  = fGeH4FGeH4/ (fGeH4FGeH4 + fSiH4FSiH4)

- Relative germane depletion

-   Into filament?

**  ****Fractional depletion of Si is 15% more than predicted from film composition**

## No Ge seems to go into filament during deposition

**Outgas 2 filaments (11.5A) in vacuum after a-SiGe:H depositions**** **

-  Observe Si (from filament) and H - but no Ge
-  Electron microprobe also finds no Ge in filament

## Depletion:  

**Easy guide to better high-rate HW a-SiGe:H?**

-   Preliminary measurements suggest low depletion of GeH4 and SiH4       indicates gas chemistry for good a-SiGe:H
-   To increase deposition rate of good a-Si:H, normally filament current (number) is increased - which also increases gas depletion
-   Plan:  Increase gas flows with growth rate to maintain low depletion

## Device fabrication with United Solar

**n-i-p device fabrication steps**
-  NREL deposits HWCVD n+ and i layers on United Solar textured substrates
-  Partial devices FedEx’ed overnight to United Solar
-  United Solar finishes solar cell
-   (deposits 0.25 cm2 p+/ITO top contacts)
-  United Solar measures device performance
- (plus QE on selected devices)

## Pure a-Si:H i-layer results (no Ge)

** ****2400Å a-Si:H i-layers**

| Voc (mV) | Jsc (mA/cm2 ) | FF |  |
| --- | --- | --- | --- |
| .96 eV | 14.08
(no QE) | .703
(.705red) | 9.50 % |

| Voc (mV) | Jsc (mA/cm2 ) | FF |  |
| --- | --- | --- | --- |
| . 93 eV | 16.13
(with QE) | .657 | 9.81 % |

**1998 best results at 16Å/s (similar United Solar collaboration)**

**noteworthy - first FF > 0.7 with this method of cell ‘fabrication’**

** ****Present results at ~ 3Å/s**

## a-SiGe:H i-layer devices (NO bandgap profiling)

| Voc (mV) | Jsc (mA/cm2) | FF |  |
| --- | --- | --- | --- |
| 545 | 22.19
(with QE) | .563
 | 6.81 % |

|  | Voc (mV) | Jsc (mA/cm2) | FF |  |
| --- | --- | --- | --- | --- |
| No bandgap
profiling | 591 | 18.09
(no QE) | .553 | 6.2% |
| V-profiled
bandgap | 625 | 20.6
(no QE) | .536 | 6.9 |

**  ****1300Å thick i-layers**

**  ****Present results (0.25 cm****2****)**

**  ****Dec 2004 results ****(0.05 cm****2****)**

## “Very preliminary” conclusions on NREL/USC device collaboration

** ****For a-Si:H cells:**
**	****highest white & red FF’s to date**
**For SiGe cells:**
** **
**	****no bandgap profiling yet**
**	****Ge content too high (want ~ 0.65 V****oc****)**
**	****device yield still not great**

**  ****We are doing better than before cleaning up O**

## Minimal N incorporated into films, even with air leak - 

**Minimal N incorporated into films, even with air leak - **
- filament does not break up N2

- SIMS O levels

- SIMS N levels

- No
- air
- leak

- Air
- leak

## SIMS vs flow rate

## Invited Jim Doyle, 

**a Ph.D. student of Alan Gallagher’s,**
** ****to come to NREL for a sabbatical**
**from Macalester College**
-  one of his tasks was to do in situ gas monitoring with
- a Mass Spectrometer to attempt to identify
- the source of the O2 contamination
- - time spent at NREL - July-Dec. 2005

## Before we ‘quit’ O,

**Dave Cohen wanted films with varying O levels to**
**examine its effect on film electronic, structural properties**

**Added an ‘air leak’ **
**(through needle valve) **
**into our system**
**to enable films **
**with varying O levels**

**Wanted to quantify this**

## Mass Spec during GeH4 flow 

**(**_**filament off**_**) identified one source of O**

-   burst of gas appears about 5 min after GeH4 flow initiated

**  ****Air leak**
**  ****peaks 28 (N****2****) and 32 (O****2****) track**
**  ****No  18 (H****2****0) peak ‘burst’**
**  ****GeH****4**** line thoroughly leak checked**
**  ****Must come from GeH****4**** tank itself**

##  Consistently now deposit a-SiGe:H with low O levels

-   Why not ruin a good thing?
- Installed intentional air leak for collaboration with U. Oregon on effect of oxygen in a-SiGe:H

- SIMS

## Film quality may correlate with gas depletion (chemistry)

**  ****Vary dep rate and flows with E****Tauc**** ~ 1.48-1.52 eV**

| Sample # | 1564 | 1575 | 1576 | 1577
Increase flows |
| --- | --- | --- | --- | --- |
| Ifil/# | 10.5A/1  | 21A/2  | 23A/2  | 23A/2  |
| f(SiH4) | 0.056 | 0.097 | 0.178 | 0.11 |
| f(GeH4) | 0.085 | 0.163 | 0.267 | 0.19 |
| Rd (Å/s) | 2.16 | 3.87 | 6.73 | ~5.80 |
| L (cm)-1 |  7.21e-6 | 4.6 e-6 | 1.7 e-6 |  3.7 e-6 |
| L/D |  1.02e4 |  3.33e3 |  3.53e3 |  3.30e3 |

**Film quality may correlate with gas depletion (chemistry)**