Consistent optical and electrical determination of carrier concentrations for the accurate modeling of the transport properties of n-type Ge

A consistent methodology is presented to extract carrier concentrations in n-type Ge from measurements of the infrared dielectric function and the Hall effect. In the case of the optical measurements, usually carried out using spectroscopic ellipsometry, the carrier concentration is affected by the doping dependence of the conductivity effective mass, which is computed using a model of the electronic density of states that accounts for non-parabolicity and is fit to electronic structure calculations. Carrier concentrations obtained from Hall measurements require a knowledge of the Hall factor, which is arbitrarily set equal to unit in most practical applications. We have calculated the Hall factor for n-Ge using a model that accounts for scattering with phonons and with ionized impurities. We show that determinations of the carrier concentration n using our computed effective mass and Hall factor virtually eliminates any systematic discrepancy between the two types of measurement. We then use these results to compute majority carrier mobilities from measured resistivity values, to compare with measurements of minority carrier mobilities, and to fit empirical expressions to the doping dependence of the mobilities that can be used to model Ge devices.Comment: 11 pages, 5 fgure

Final Report

The project addressed the need for improved multijunction solar cells as identified within the Solar America Initiative program. The basic Ge/InGaAs/InGaP triple-junction structure that has led to record commercial efficiencies remains unoptimized due to excess current in the germanium component. Furthermore, its deployment cannot be scaled up to terawatt-level applications due to bottlenecks related to germaniumâÂÂs cost and abundance. The purpose of the program was to explore new strategies developed at Arizona State University to deposit germanium films on much cheaper silicon substrates, largely eliminating the germanium bottleneck, and at the same time to develop new materials that should lead to an improvement in multijunction efficiencies. This included the ternary alloy SiGeSn, which can be inserted as a fourth junction in a Ge/SiGeSn/InGaAs/InGaP structure to compensate for the excess current in the bottom cell. Moreover, the possibility of depositing materials containing Sn on Si substrates created an opportunity for replacing the bottom Ge cell with a GeSn alloy, which, combined with new III-V alloys for the top cells, should enable 4-junction structures with perfectly optimized band gaps. The successes of the program, to be described below, has led to the developments of new strategies for the growth of high-quality germanium films on Si substrates and to a widespread recognition that SiGeSn is likely to play a significant role in future generations of high-efficiency devices, as demonstrated by new research and intellectual property efforts by major US industrial players

Radiation-induced Electron and Hole Traps in Ge\u3csub\u3e1-x\u3c/sub\u3eSn\u3csub\u3ex\u3c/sub\u3e (x = 0-0.094)

The band structure of germanium changes significantly when alloyed with a few percent concentrations of tin, and while much work has been done to characterize and exploit these changes, the corresponding deep-level defect characteristics are largely unknown. In this paper, we investigate the dominant deep-level defects created by 2 MeV proton irradiation in Ge1 -xSnx (x = 0.0, 0.020, 0.053, 0.069, and 0.094) diodes and determine how the ionization energies of these defects change with tin concentrations. Deep-level transient spectroscopy measurements approximate the ionization energies associated with electron transitions to/from the valence band (hole traps) and conduction band (electron traps) in the intrinsic regions of p-i-n diode test structures. The prominent deep-level hole traps may be associated with divacancies, vacancy-tin complexes, and vacancy-phosphorous complexes (V2, V-Sn, and V-P, respectively), with the presumed V-P hole trap dominating after room temperature annealing. The ionization energy level of this trap (approximated by the apparent activation energy for hole emission) is close to the intrinsic Fermi level in the 0% and 2% Sn devices and decreases as the tin concentration is increased, maintaining an approximately fixed energy spacing below the indirect conduction band edge. The other hole traps follow this same trend, and the dominant electron trap ionization energies remain roughly constant with changes in tin concentrations, indicating they are likewise pinned to the conduction band edge. These results suggest a pattern that may, in many cases, apply more generally to deep-level defects in these alloys, including those present in the as-grown materials

Temperature-dependent photoluminescence of Ge/Si and Ge 1-ySn y/Si, indicating possible indirect-to-direct bandgap transition at lower Sn content

Temperature (T)-dependent photoluminescence (PL) has been investigated for both p-Ge and n-Ge1-ySny films grown on Si substrates. For the p-Ge, strong direct bandgap (ED) along with weak indirect bandgap related (EID) PL at low temperatures (LTs) and strong ED PL at room temperature (RT) were observed. In contrast, for the n-Ge1-ySny, very strong dominant EID PL at LT and strong ED PL were observed at RT. This T-dependent PL study indicates that the indirect-to-direct bandgap transitions of Ge1-ySny might take place at much lower Sn contents than the theory predicts, suggesting that these Ge1-ySny could become very promising direct bandgap semiconductors

Observation of Heavy- and Light-hole Split Direct Bandgap Photoluminescence from Tensile-strained GeSn (0.03% Sn)

Temperature- (T-) and laser power-dependent photoluminescence (PL) measurements have been made for the tensile-strained, undoped GeSn (0.03% Sn) film grown on Si substrate. The PL results show not only clear strain-split direct bandgap transitions to the light-hole (LH) and heavy-hole (HH) bands at energies of 0.827 and 0.851 eV at 10 K, respectively, but also clearly show both strong direct and indirect bandgap related PL emissions at almost all temperatures, which are rarely observed. This split of PL emissions can be directly observed only at low T and moderate laser power, and the two PL peaks merge into one broad PL peak at room temperature, which is mainly due to the HH PL emission rather than LH transition. The evolution of T-dependent PL results also clearly show the competitive nature between the direct and indirect bandgap related PL transitions as T changes. The PL analysis also indicates that the energy gap reduction in Γ valley could be larger, whereas the bandgap reduction in L valley could be smaller than the theory predicted. As a result, the separation energy between Γ and L valleys (∼86 meV at 300 K) is smaller than theory predicted (125 meV) for this Ge-like sample, which is mainly due to the tensile strain. This finding strongly suggests that the indirect-to-direct bandgap transition of Ge1−ySny could be achieved at much lower Sn concentration than originally anticipated if one utilizes the tensile strain properly. Thus, Ge1−ySny alloys could be attractive materials for the fabrication of direct bandgap Si-based light emitting devices

Degenerate Parallel Conducting Layer and Conductivity Type Conversion Observed from \u3ci\u3ep\u3c/i\u3e-Ge\u3csub\u3e1 - y\u3c/sub\u3eSn\u3csub\u3ey\u3c/sub\u3e (y = 0.06%) Grown on \u3ci\u3en\u3c/i\u3e-Si Substrate

Electrical properties of p-Ge1−ySny (y = 0.06%) grown on n-Si substrate were investigated through temperature-dependent Hall-effect measurements. It was found that there exists a degenerate parallel conducting layer in Ge1−ySny/Si and a second, deeper acceptor in addition to a shallow acceptor. This parallel conducting layer dominates the electrical properties of the Ge1−ySny layer below 50 K and also significantly affects those properties at higher temperatures. Additionally, a conductivity type conversion from p to n was observed around 370 K for this sample. A two-layer conducting model was used to extract the carrier concentration and mobility of the Ge1−ySny layer alone

Complementary Metal-oxide Semiconductor-compatible Detector Materials with Enhanced 1550 nm Responsivity via Sn-doping of Ge/Si(100)

Previously developed methods used to grow Ge1−ySny alloys on Si are extended to Sn concentrations in the 1019−1020 cm−3 range. These concentrations are shown to be sufficient to engineer large increases in the responsivity of detectors operating at 1550 nm. The dopant levels of Sn are incorporated at temperatures in the 370–390 °C range, yielding atomically smooth layers devoid of threading defects at high growth rates of 15–30 nm/min. These conditions are far more compatible with complementary metal-oxide semiconductor processing than the high growth and processing temperatures required to achieve the same responsivity via tensile strain in pure Ge on Si. A detailed study of a detector based on a Sn-doped Ge layer with 0.25% (1.1 × 1020 cm−3) Sn range demonstrates the responsivity enhancement and shows much better I-V characteristics than previously fabricated detectors based on Ge1−ySny alloys with y = 0.02

Advanced Semiconductor Materials for Breakthrough Photovoltaic Applications

The project addressed the need for improved multijunction solar cells as identified within the Solar America Initiative program. The basic Ge/InGaAs/InGaP triple-junction structure that has led to record commercial efficiencies remains unoptimized due to excess current in the germanium component. Furthermore, its deployment cannot be scaled up to terawatt-level applications due to bottlenecks related to germaniumâÂÂs cost and abundance. The purpose of the program was to explore new strategies developed at Arizona State University to deposit germanium films on much cheaper silicon substrates, largely eliminating the germanium bottleneck, and at the same time to develop new materials that should lead to an improvement in multijunction efficiencies. This included the ternary alloy SiGeSn, which can be inserted as a fourth junction in a Ge/SiGeSn/InGaAs/InGaP structure to compensate for the excess current in the bottom cell. Moreover, the possibility of depositing materials containing Sn on Si substrates created an opportunity for replacing the bottom Ge cell with a GeSn alloy, which, combined with new III-V alloys for the top cells, should enable 4-junction structures with perfectly optimized band gaps. The successes of the program, to be described below, has led to the developments of new strategies for the growth of high-quality germanium films on Si substrates and to a widespread recognition that SiGeSn is likely to play a significant role in future generations of high-efficiency devices, as demonstrated by new research and intellectual property efforts by major US industrial players