# Presentation: 130123

## Phase Diagrams for  Si1-xGex:H Thin Films

**Contributors**
**N. J. Podraza ****(1)****,  ****C. R. Wronski ****(2)****, M. W. Horn ****(2)****, **
**and R. W. Collins ****(1)**
- (1)  Department of Physics and Astronomy,
- The University of Toledo, Toledo, OH, USA
- (2)  Materials Research Institute,
- The Pennsylvania State University, University Park, PA, USA

## Outline:

- Experimental details:
- 		Real time spectroscopic ellipsometry (RTSE)
- III.	Experimental details:
- 		Plasma-enhanced chemical vapor deposition (PECVD)
- IV.	Comparison of Si1-xGex:H phase diagrams
- 		vs. H2-dilution flow ratio for PECVD films prepared on 	the anode and cathode at 200C
- V.	Comparison of Si1-xGex:H phase diagrams vs.
- 		H2-dilution flow ratio at different GeH4 flow ratios
- VI.	Comparison of Si:H and Si1-xGex:H phase diagrams vs. 	Helium dilution
- VII.	Summary and future directions

**Outline****:**

## I.  Motivation

- The deposition principle for the a-Si:H i-layer is to employ the highest possible H2-dilution flow ratio R=[H2]/[SiH4] without crossing the (amorphous)-to-(mixed-phase) transition [a (a+c)].
- A second principle is to ensure the                       largest possible thickness for the onset of the amorphous roughening transition [a  a]                               as assessed in studies on c-Si substrates.
- Similar optimization principles are expected for Si1-xGex:H alloys.
- Si1-xGex:H alloys are being studied using RTSE to locate optimum parameter space regions which are then verified by other analysis methods.

-   We seek to optimize the components of a-Si:H-based solar cells that apply 	triple junction n-i-p design based on an understanding of growth process.

## Experimental Details:  PECVD of Si1-xGex:H for Phase Diagram Development

- Same single chamber rf PECVD system as that used previously for extensive studies of Si:H phase diagrams
- (Native-oxide)/c-Si substrates for smoothness and highest sensitivity to amorphous roughening transitions
- Minimum rf plasma power for a stable plasma (0.08 mW/cm2)
- Low partial pressure of [SiH4]+[GeH4] (~0.06 Torr) with a total pressure < 1.0 Torr for all depositions versus H2-dilution flow ratio
- H2 flow ratio, R=[H2]/{[SiH4]+[GeH4]} as the abscissa of the phase diagram which controls the phase of the film (a, a+c, c)
- Variable GeH4 flow ratio, G=[GeH4]/{[SiH4]+[GeH4]}
- ►	yields room temperature optical (Tauc) gap of
- 		Eg~1.8  1.3 eV     for G = 0  0.167
- ►	also controls phase of the film (a, a+c, c)
- Anode (grounded) and cathode (20 V self-bias) electrode config's
- Variable He flow ratio, He=[He]/{[SiH4]+[GeH4]} controls the phase of the film (a, a+c, c)

##  IV.  Explanation of 

**	****a ******** a Roughening Transition**

##  IV.  Explanation of a  a Roughening Transition

**a ******** a transitions**

**PECVD parameters for Si:H series:**
- Substrate: c‐Si;
- T = 200C;
- P = 0.72 W/cm2 ;
- R = [H2]/[SiH4]
- 	~15 - 60;
- ptot = 3 Torr

- Fit to experimental data for a-Si:H with R=40 using the 1-D continuum model.  In this model, initial nuclei with a mean radius of 24 Å are assumed, along with a diffusion length of 0=45 Å.

- Amorphous roughening transition thickness obtained from real time SE and the critical diffusion length obtained from the 1-D model plotted vs. R for the series of a-Si:H samples.
- R and aa are linearly related

## IV.	Explanation of the a  (a+c) Roughening

- 	Conversion of RTSE data to
- 	simple geometric parameters
**Assumptions:**
- (i)	Abrupt onset of  c nucleation versus thickness
- (ii)	Nuclei on a square grid
- (iii)	Constant cone angle for the preferential growth of  c’s
- (iv)	Linearly increasing radius of cone caps
**Approach:**
- From geometric considerations:
- db = db,coal  db,trans    and
- ds = ds,coal  ds,trans       together
**yield  (N****d****, ********)****	**

**IV.****	****Explanation of the a ******** (a+********c) Roughening**
**  ****	****and the (a+********c) ******** ********c Smoothening Transitions**** **

**Geometrical model for ****(a+********c)-Si:H**** phase evolution**

## IV. Explanation of : Width of the Dielectric Function 

- Monotonic increase in width   indicates a reduction in excited state lifetime  ~ *h*/ due to decreased order with alloying.
- Using a simple expression for mean free path *L*=*h*/ (: electron velocity):
- :  2  3 eV    *L*:  5  4 Å ,  and a lack of long-range order.

- Fit using

- expression
- from
- A.S. Ferlauto
- et al.
- J. Appl. Phys.
**92**, 2424
- (2002).

## IV. Comparison of Si1-xGex:H Anode/Cathode Diagrams

**G=0.167**

- a  (a+c) transition is shifted to much higher R for **cathode**
- 	**Si****1-x****Ge****x****:H **
- 2)  This opens up a narrow window 70  R  90 that yields an        **a ******** a transition at a much higher value of d****b**** ~ 2000 ****Å** for **cathode**** **Si1-xGex:H
- 	****** ****high surface stability**

- Higher deposition rates and higher a(a+c) thicknesses (at db = 4000 Å) for **cathode**    **Si****1-x****Ge****x****:H**** ****as compared to** **anode** **Si****1-x****Ge****x****:H **under otherwise similar conditions;
- here **G = [GeH****4****]/{[SiH****4****]+[GeH****4****]} = 0.167**

## V. 	Optimization of Si1-xGex:H Cathode PECVD at	Different GeH4 Flow Ratios

- Higher deposition rates with increasing GeH4 flow ratio, **G = [GeH****4****]/{[SiH****4****]+[GeH****4****]}** at **R=10**
- For fixed **G**, **** decreases up to maximal **R** prior to a(a+c)
- a  a transition at maximum bulk layer thickness shifts to higher **R** with increasing **G**

## VI.  Comparison of Si:H with Helium Dilution

**VI.  Comparison of Si:H with Helium Dilution**

- Comparison of the growth of **He=0** R=0 and **R=10** **a-Si:H** films with a **He=10** R=0 film:
-  	a  a transition shifts to higher bulk 	layer thickness between **He=0** and 	**He=10** R=0 films, but fails to reach 	improvement gained by **R=10**
- 	H2 dilution

- Comparison of the growth of R=40
**Si:H films** deposited with **He=0** and **He=10**:
-  	overall lower roughness amplitude
-  	a  (a+c) transition occurs at
- 	db < 10 Å for **He=0**, and
- 	db = 1400 Å for **He=10**
- 	 He suppresses microcrystallinity

## VI.  Extended Si:H Phase Diagram with He dilution

- a  (a+c) transition is shifted to higher bulk layer thickness for **He=10 ****Si:H **allowing for an extended protocrystalline regime
- a  a transition is shifted to higher bulk layer thickness, indicating minor improvements in film quality

- Higher deposition rates with **He=10** **Si****:****H**** ****as compared to** **He=0** **Si:H **under otherwise similar conditions;
- here **He = [He]/{[SiH****4****]}**

**VI.  Extended Si:H Phase Diagram with He dilution**

**4000 ****Å**

## VII. 	Comparison of Si1-xGex:H with He dilution

**VII. ****	****Comparison of Si****1-x****Ge****x****:H with He dilution**

- Comparison of the growth of **He=0**** **and** ****He=10**** ****a-Si****1-x****Ge****x****:H **films with R=10** **:
-  	a  a transition shifts to higher bulk 	layer thickness between **He=0** and 	**He=10** R=10 films

- Comparison of the growth of **He=0** and **He=20** **Si****1-x****Ge****x****:H films** with R=80 :
-  	a  a transition shifts to higher bulk 	layer thickness
-  	a  (a+c) transition shifts to > 3500 Å 	 He suppresses microcrystallinity

- G=0.167

- G=0.167

## VII.	Extended Si1-xGex:H Phase Diagrams with He dilution

**R=10** series a  a transition is shifted to higher bulk layer thickness initially but saturates at db=200 Å
**R=80** series a  a transition is shifted to higher bulk layer thickness while the a  (a+c) transition is strongly suppressed

**VII.****	****Extended Si****1-x****Ge****x****:H Phase Diagrams with He dilution**

**G=0.0167**

**4000 ****Å**

- Higher deposition rates for
**R=10** **Si****1-x****Ge****x****:H** films up to He=10, but decreasing with total dilution S
- Slightly increasing depostion rate for
**R=80** **Si****1-x****Ge****x****:H**  with He=20

## Phase Diagrams for Si1-xGex:H Thin Films

- A.	Si:H and Si1-xGex:H phase diagrams have been developed at T=200oC,
- 	comparing the anode and cathode electrode configurations and the role of
- 	low-energy ion bombardment.  These comparisons show that:
- 	1)  the a(a+c) transitions shift to higher R for films deposited on the cathode
- 			for both Si:H and Si1-xGex:H
- 		a. for Si:H, this shift creates an extended protocrystalline regime at higher
- 			dilution levels, while maintaining very high a  a transition thicknesses
- 			at lower dilutions
- 		b. for Si1-xGex:H, this shift opens a narrow window leading to a  a transitions
- 			at much higher bulk thicknesses, suggesting significant increases in
- 			precursor surface diffusion for a-Si1-xGex:H deposited at the cathode
- 	2)  The higher surface stability for the cathodic films is accompanied by
- 	     	smoother surfaces

**Phase Diagrams for ****Si****1-x****Ge****x****:H Thin Films**

## VII.	  Summary (continued)

- B.	Si1-xGex:H phase diagrams versus R have been developed at T=200oC under
- 	cathodic deposition conditions for different values of the GeH4 flow ratio G.
- 	This comparison shows that:
- 	1)  the aa transitions shift to higher thicknesses with increasing R right up
- 		to the a(a+c) transitions, indicating the beneficial effect
- 		of H on precursor surface diffusion under all alloying conditions
- 	2)  the dielectric function broadening parameter  decreases with increasing H2
- 		dilution up to the a(a+c) transition, indicating improved film quality
- Si:H and Si1-xGex:H (G=0.167) have been studied with He dilution.
- 	1)  The presence of He in both a-Si:H and a-Si1-xGex:H has been shown to
- 		shift the aa roughening transition to higher bulk layer thicknesses,
- 		indicating enhanced surface diffusion, but the effect is weaker than for H2
- 	2)  Similarly, He has been shown to suppress microcrystalline formation,
- 		shifting the a(a+c) transition in thin film Si:H and Si1-xGex:H to
- 		higher bulk layer thickness, creating an extended protocrystalline regime

**Phase Diagrams for ****Si****1-x****Ge****x****:H Thin Films**

## Phase Diagrams for Si1-xGex:H Thin Films

- Explore additional non-standard deposition techniques to
- 	improve film quality and/or increase rate
- Controlled low-energy ion bombardment
- Total pressure variation
- Vhf plasma excitation frequencies
- Preparation and optimization of graded layer a-Si:H,
- 	a-Si1-xGex:H, and c-Si:H structures through
- 	simultaneous variation in G, R, and He

**Phase Diagrams for ****Si****1-x****Ge****x****:H Thin Films**

## Slide 18

## Outline

- I.	Statement of the Problem
- II.	Review of Deposition Phase Diagrams
- III.	Instrumentation Unique to Univ. Toledo
- IV.	Approaches for Real Time Analysis
- 	1.	Rigorous Virtual Interface Analysis
- 	2.	Empirical E2 Amplitude Analysis
- V.	Comparison of Approaches
- VI.	Results for One-Step Microcrystalline Si:H
- VII.	Results for Two-Step Microcrystalline Si:H
- VIII.	Summary and Future

## Statement of Problem:  Optimization of i-layer for c-Si:H Solar Cells

**A. Shah et al.  Sol. Energy Mater. & Sol. Cells 78, 469 (2003)**
- "If we add hydrogen to the plasma, ... the layer remains amorphous until we
- reach a threshold concentration. So far, the best microcrystalline solar cells
- are deposited near the threshold concentration; for very high values of hydrogen
- dilution, the material does not appear to be usable for solar cells, because of the
- cracks ... that appear also to act as channels by which contamination ...
- can enter into the layer."
**J. Bailat, Ph.D. Thesis (2004) Neuch****â****tel:  **
- "Amorphous tissue is responsible for the passivation of the nano-crystals surface ..."

## Review of Phase Diagrams and Optimization Procedures 

**Question:****	**The substrates are R=0 a-Si:H films; however, once the overlying film begins to 	grow in the first step, a new "substrate" is established, either R=40 a-Si:H or R=200 	c-Si:H.  Then why should the same diagram be valid for multistep deposition?
**Answer:****	**The same phase diagram is NOT valid and in both cases the phase 	boundary 	shifts to lower R in subsequent deposition steps.  This effect must be taken into 	account in the optimization prescription.
**It is important to analyze in greater detail the substrate dependence of the phase evolution including the effect of crystallite size. **

## Unique Instrumentation for RTSE and Graded Solar Cell Fabrication

| Table I. AM1.5 IV data for VHF 3 Å/s c-Si:H single-junction solar cells
using H2 dilution grading for i-layer deposition |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Sample Number | H2-dilution grading | 
 (%) | Voc
(V) | Jsc
(mA/cm2) |  red Jsc
(mA/cm2) 
(650-1000nm) | FF | Rs
(Ohm*cm2) |
| GD1955 | profile #4  | 8.02 | 0.500 | 22.9 | 8.72 | 0.701 | 3.20 |

-  	H2 dilution grading has a great effect on c-Si:H cell performance.
-  	The film growth processes of sample GD1955 were measured using RTSE.

## Approaches for Real Time Analysis 

**PECVD conditions:**
-  Substrate temperature:	T  	=	200C
-  Plasma frequency:		f  	=	60 MHz (VHF)
-  Plasma power density:	P  	=	0.08 mW/cm2
-  Hydrogen dilution :	  	R 	=	[H2]/[SiH4]  =  20
-  Total gas pressure:		ptot	=	0.54 Torr
**Four growth regimes:**
- I:	d < 100Å:	a-Si:H nucleation/coalescence
- II:	100Å < d < 500Å:	microcrystal nucleation from
- 		amorphous phase;
- 		(a+c)-Si:H growth
- III:	500Å < d < 1500Å:	microcrystal coalescence
- IV:  d > 1500Å:        	single-phase c-Si:H growth

**Two different approaches were applied to deduce **
**the volume fraction evolution throughout the growth:**
- 1)  Rigorous virtual interface analysis. Advantages: 	high accuracy & depth resolution
- 	Disadvantages:	difficult to implement in real time
- 2)  Empirical E2 amplitude analysis:	Advantages: 	easy to implement in real time
- 	**after C. Ross et al. MRS (2003)**	Disadvantages:	indirect -- requires calibration
- 			low depth resolution

## Comparison of Approaches

**	****Depth profiles are from:**
-  	_Rigorous VI analysis_ (red circles):
-     (i)	a-Si:H dielectric function 			obtained from
- 		{(E, t), (E, t) in first 120 Å.
-     (ii)	c-Si:H dielectric function 			obtained from
- 		{(E), (E)}at a thickness of
-                 ~ (1500 Å).
-     (iii) {ds(t), fc(t), tt  t  tc } 			determined from VI analysis; 			r(t) can be fixed at value
- 		obtained in uniform growth 			analysis.
-  	_Empirical E__2__ amplitude analysis_ of 		(E) (blue circles):
-     	performed as described on 			previous viewgraph

**Differences in profiles are attributed to **
**differences in depth resolution ********d**
-     Rigorous VI analysis (solid circles):	d = r t  20 Å
-     Empirical analysis (open circles):  	d  2[ (4.05 eV)]-1  2/(1.55x10–6) cm  130 Å

## Results for One-Step c-Si:H

**Cone model of d****s****(t) vs. d****b****(t)**
- Model is used to extract nucleation density Nd and cone angle .
- Assumptions of the model:
-  	All nuclei originate at d = db,t;
-  	Nuclei grow with cone angle ;
-  	Tops of cones are spherical caps 			with radius d  db,t.

**Correlation of N****d**** and ******** with********d****b,t**
-  	Cone angle nearly constant at
- 15-20° ;
-  	Nucleation densitydecreases rapidly
- 		with increasing db,t ;
**Good agreement with AFM and XTEM.**

- Increasing
-        R

## Results for Two-Step c-Si:H

**Results for Two-Step ********c-Si:H**

**Deposition Procedure:**
- Substrate R=0 a-Si:H
- (to simulate a-Si:H n-layer)
- Deposit Si:H with R=40
- (no seeding);
- this leads to a 200 Å amorphous (protocrystalline) layer before the a  (a+c) transition
- Reduce R to R=20 when fc
- reaches 0.5 to slow the rate
- of crystallite growth

**Modeling the Crystallite Evolution in Two Steps**
- Crystallite density can be estimated by fitting fc versus db
- in the initial stages of growth or the final stages of coalescence using the cone growth model.
- The crystallite density is found to decrease with decreasing R.

**R=0 a-Si:H**

**R=40 a-Si:H**

**R=20 a-Si:H**

## Summary 

- Amorphous phase passivation at the grain boundaries of c-Si:H
- i-layers appears necessary for the fabrication of optimum solar cells.
- This requires depositing the film near the boundary between amorphous and mixed-phase [(a+c)-Si:H] film growth.
- 	Because the boundary between amorphous and mixed-phase
- [(a+c)-Si:H] film growth shifts to decreasing R with increasing thickness, it becomes necessary to grade the H2-dilution during the growth of c-Si:H for optimum i-layer material.
- 	However, because of its high substrate sensitivity, the deposition phase diagram provides only qualitative guidance for identifying the optimum graded layer process.
- 	As a result, real time characterization and control of graded layer growth is important.  We have compared two procedures for determining the depth profile in the volume fraction of crystallinity.  Both yield reasonable results and are promising for real time control.