D. Slobodin

Photothermal and photoconductive determination of surface and bulk defect densities in amorphous silicon films

The sub‐band‐gap optical absorption spectra of high‐quality hydrogenated amorphous silicon (a‐Si:H) films are shown to be dominated by surface and interface state absorption when measured by photothermal deflection spectroscopy (PDS), while spectra determined using the constant photocurrent method (CPM) are not. For bulk defect states (both as‐deposited and light‐induced), the integrated subgap absorption is approximately twice as large for PDS as for CPM. Similarly, the conversion factor relating integrated subgap absorption with neutral dangling bond density is twice as large for CPM as PDS. This factor of 2 results from CPM seeing only transitions from below midgap into the conduction band while PDS sees transitions from the valence band into states above midgap as well.

Properties of a‐Si,Ge:H,F alloys prepared by rf glow discharge in an ultrahigh vacuum reactor

a‐Si1−xGex: H,F alloys with the Ge concentration x ranging from 0 to 1 have been prepared by the rf glow discharge decomposition of SiF4, GeF4, and H2 gas mixtures. An ultrahigh vacuum deposition system has been designed and constructed for the preparation of these alloys. The stainless‐steel deposition chamber has Cu gasket‐sealed flanges, is turbomolecular pumped, and reaches a base pressure below 10−7 Torr (10−5 Pa). This deposition system incorporates a load lock which permits short pump‐down cycles and which may reduce impurity contamination of the films. The system is described in detail. Compositional, structural, optical, and electronic properties of alloy films made with this system are reported.

Amorphous hydrogenated silicon studied by positron lifetime spectroscopy

Hydrogenated amorphous silicon (a-Si:H) prepared by dc glow discharge in silane was investigated by positron lifetime measurements at room temperature. The lifetime spectrum shows considerably longer lifetimes than in simultaneously measured Si single crystals. The dominant component with the time constantτ2=402 ps is discussed thoroughly in conjunction with positron trapping at microvoids containing more than 10 to 15 vacancies. Positron trapping at H-saturated dangling bonds cannot be ruled out. The long-lived component withτ3=1800 ps (I3=0.06) indicates positronium formation at larger voids.

Differential surface photovoltage measurement of minority‐carrier diffusion length in thin films

A new surface photovoltage technique is introduced for the determination of the minority‐carrier diffusion length in thin semiconductor films. The technique eliminates the effect of a back surface barrier on the measurement by separating out the back barrier contribution to the total photovoltage signal. With a direct difference method based on a double lock‐in technique, the measurement can be carried out in a single run.

Sub‐bandgap optical absorption and light‐induced defects in amorphous silicon

Photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM) are used to monitor the changes in density of deep defects associated with the Staebler‐Wronski effect. These values are correlated with the electron spin resonance (ESR) signal, dc photoconductivity, and minority‐carrier μτ values determined from bias‐dependent carrier collection efficiency studies. The density of deep defects determined by subgap absorption grows with light‐soaking time t following the same functional form (t1/3) as the ESR signal. The initial and light‐induced defects have similar carrier recombination cross‐sections.

a-(Si, Ge):H, F Alloys Prepared from SiH4 and GeF4

The properties of a-(Si, Ge):H, F alloys prepared by glow discharge deposition from SiH 4 and GeF 4 are described. The measured IR absorption spectra, sub-gap absorption spectra, dark conduction activation energies, carrier drift mobilities and deep trapping lifetimes of these alloys are similar to those of alloys prepared from SiF 4 , GeF 4 , and H 2 . However, they have over an order of magnitude lower photoconductivity over most of the composition range. Infrared absorption measurements indicate that these alloys have a fluorine content less than 1.5 at.% and a Si-H 2 content that increases with germanium concentration.

Reply to the comment by S. Dannefaer et al. on the paper of H. E. Schaefer et al. on “amorphous hydrogenated silicon studied by positron lifetime spectroscopy”

In the original paper by Schaefer et al. the assignment of positron lifetimes to monovacancies and vacancy agglomerates in Si has been discussed in order to interpret the lifetime of 402 ps observed in amorphous hydrogenated Si. In the present reply to the comment of Dannefaer, Mascher and Kerr we will demonstrate that a precise and critical discussion of the available data is indispensible.

Infrared Spectroscopy of Deuterated a-Si, Ge:D, F Alloys Prepared by DC Glow Discharge Deposition

Deuterated and fluorinated amorphous silicon-germanium alloys, a-Si, Ge:D, F, were studied by Fourier transform infrared (IR) spectroscopy. No Ge.-F modes are observed. The intensity of the Si-F and Si-F 2 modes increases with Ge concentration. So does thae intensity of SiF 4 which is trapped as isolated molecules. No DF (IR) or F 2 (Raman) is observed. The IR spectra of alloys annealed at 300, 400, 500 and 600° C show that the fluorine in the Si-F and Si-F 2 groups and in the SiF 4 molecules is in thermochemical equilibrium.

Light‐induced defect generation and thermal healing in amorphous silicon‐germanium alloys

The electronic and optical properties of a‐Si,Ge:H,F alloys are investigated for the initial (as‐grown) state as well as after annealing, light soaking, and subsequent annealing. The optical absorption spectra were determined from optical transmission, the constant photocurrent technique and photothermal deflection spectroscopy. The photoconductivity and the temperature dependence of the dark conductivity were measured. A post‐deposition anneal was found to improve the measured properties for some of the alloys. The properties of the high gap, Eopt≳1.4 eV, and the low gap, Eopt<1.4 eV, alloys change differently with light soaking. For the high gap alloys, there is an increase in the subgap absorption, a decrease in the photoconductivity and the dark conductivity and an increase in the activation energy with light exposure. The low gap alloys show increasing subgap absorption with light soaking, however, the photoconductivity remains constant and the dark conductivity increases and its activation energy de...

The dielectric constants of a-Si,Ge:H,F alloys

Abstract We report the dielectric constants for amorphous, hydrogenated and fluorinated, silicon-germanium alloys. The alloys were prepared by d.c. glow-discharge deposition from mixtures of SiF 4 or SiH 4 , GeF 4 and H 2 . The dielectric constants were determined from the 1 MHz capacitance of Al/alloy/Cr structures. For optical gaps E opt ranging from 1.27 to 1.65 eV the dielectric constant ϵ follows the relation: ϵ = 29.7 − 9.7 E opt (eV) ±1.6.