Hole-initiated Multiplication Research Articles

Channel-Length-Modulated Avalanche Multiplication in Ambipolar WSe2 Field-Effect Transistors.

Recently there has been growing interest in avalanche multiplication in two-dimensional (2D) materials and device applications such as avalanche photodetectors and transistors. Previous studies have mainly utilized unipolar semiconductors as the active material and focused on developing high-performance devices. However, fundamental analysis of the multiplication process, particularly in ambipolar materials, is required to establish high-performance electronic devices and emerging architectures. Although ambipolar 2D materials have the advantage of facile carrier-type tuning through electrostatic gating, simultaneously allowing both carrier types in a single channel poses an inherent difficulty in analyzing their individual contributions to avalanche multiplication. In ambipolar field-effect transistors (FETs), two phenomena of ambipolar transport and avalanche multiplication can occur, and both exhibit secondary rise of output current at high lateral voltage. We distinguished these two competing phenomena using the method of channel length modulation and successfully analyzed the properties of electron- and hole-initiated multiplication in ambipolar WSe2 FETs. Our study provides a simple and robust method to examine carrier multiplication in ambipolar materials and will foster the development of high-performance atomically thin electronic devices utilizing avalanche multiplication.

Impact ionization coefficients and critical electric field in GaN

Avalanche multiplication characteristics in a reverse-biased homoepitaxial GaN p–n junction diode are experimentally investigated at 223–373 K by novel photomultiplication measurements utilizing above- and below-bandgap illumination. The device has a non-punch-through one-side abrupt p–-n+ junction structure, in which the depletion layer mainly extends to the p-type region. For above-bandgap illumination, the light is absorbed at the surface p+-layer, and the generated electrons diffuse and reach the depletion layer, resulting in an electron-injected photocurrent. On the other hand, for below-bandgap illumination, the light penetrates a GaN layer and is absorbed owing to the Franz–Keldysh effect in the high electric field region (near the p–n junction interface), resulting in a hole-induced photocurrent. The theoretical (non-multiplicated) photocurrents are calculated elaborately, and the electron- and hole-initiated multiplication factors are extracted as ratios of the experimental data to the calculated values. Through the mathematical analyses of the multiplication factors, the temperature dependences of the impact ionization coefficients of electrons and holes in GaN are extracted and formulated by the Okuto–Crowell model. The ideal breakdown voltage and the critical electric field for GaN p–n junctions of varying doping concentration are simulated using the obtained impact ionization coefficients, and their temperature dependence and conduction-type dependence were discussed. The simulated breakdown characteristics show good agreement with data reported previously, suggesting the high accuracy of the impact ionization coefficients obtained in this study.

Geiger-Mode Operation of AlGaN Avalanche Photodiodes at 255 nm

We report the Geiger mode operation of back-illuminated AlGaN avalanche photodiodes. The devices were fabricated on transparent AlN templates specifically for back-illumination to leverage hole-initiated multiplication. The spectral response was analyzed with a peak detection wavelength of 255 nm with an external quantum efficiency of ~14% at zero bias. Low-photon detection capabilities were demonstrated in devices with areas 25 μm x 25 μm. Single photon detection efficiencies of ~5% were achieved.

Mean multiplication gain and excess noise factor of GaN and Al0.45Ga0.55N avalanche photodiodes

In this work, Monte Carlo model is developed to investigate the avalanche characteristics of GaN and Al0.45Ga0.55N avalanche photodiodes (APDs) using random ionization path lengths incorporating dead space effect. The simulation includes the impact ionization coefficients, multiplication gain and excess noise factor for electron- and hole-initiated multiplication with a range of thin multiplication widths. The impact ionization coefficient for GaN is higher than that of Al0.45Ga0.55N. For GaN, electron dominates the impact ionization at high electric field while hole dominate at low electric field whereas Al0.45Ga0.55N has hole dominate the impact ionization at higher field while electron dominate the lower field. In GaN APDs, electron-initiated multiplication is leading the multiplication gain at thinner multiplication widths while hole-initiated multiplication leads for longer widths. However for Al0.45Ga0.55N APDs, hole-initiated multiplication leads the multiplication gain for all multiplication widths simulated. The excess noise of electron-initiated multiplication in GaN APDs increases as multiplication widths increases while the excess noise decreases as the multiplication widths increases for hole-initiated multiplication. As for Al0.45Ga0.55N APDs, the excess noise for hole-initiated multiplication increases when multiplication width increases while the electron-initiated multiplication increases with the same gradient at all multiplication widths.

Simulation study of front-illuminated GaN avalanche photodiodes with hole-initiated multiplication

A flip-chip GaN p-i-n-i-n avalanche photodiode (APD) which integrates the merits of the prevailing APDs—hole-initiated multiplication process and front-illumination is proposed and studied via simu...

Avalanche characteristics in thin GaN avalanche photodiodes

A Monte Carlo model using random ionization path lengths describing the carriers quantum transport in thin gallium nitride (GaN) avalanche photodiodes (APDs) for ultraviolet detection in industry is developed. This work simulated avalanche characteristics such as impact ionization coefficients, mean multiplication gain and excess noise factor of GaN APDs at 0.05 μm, 0.1 μm, 0.2 μm and 0.3 μm multiplication widths in an electric field. The model simulates higher electron impact ionization coefficients than that of the hole for an electric field greater than 4.04 MV cm−1. Mean multiplication gain and excess noise factor are simulated based on the electric field dependent impact ionization coefficients. Our results show that electron-initiated multiplication gives higher multiplication gain and lower excess noise than hole-initiated multiplication for a multiplication width below 0.3 μm. Devices with dead space in general give a lower excess noise.

Germanium/Silicon-Germanium Heterostructure Avalanche Photodiodes on Silicon

Near-infrared photodiodes (PDs) of Ge on Si have been widely studied in Si photonics for the optical communications (1.3–1.6 μm). Ge-based avalanche PDs (APDs) have been also studied for sensitive photodetections. There are mainly two different types of Ge APDs on Si. One utilizes Ge as the multiplication layer, i.e., Ge homojunction APDs [1], while the other one utilizes Si, i.e., Ge/Si heterojunction APDs [2]. Si is one of the best materials for the multiplication layer in terms of low excess noises, although relatively large applied voltage more than 20 V, corresponding to the electric field strength more than 300 kV/cm, is required to obtain the multiplication gain due to the small ionization coefficients. In order to realize low-noise and low-voltage Ge APDs, we have recently proposed a new structure utilizing Ge/graded-SiGe heterojunction multiplication layer [3]. As in Fig. 1(a), photo-generated holes are injected via the graded SiGe to the Ge multiplication layer. The band discontinuity at the graded-SiGe/Ge interface can enhance the impact ionization due to the sudden increase of kinetic energy across the interface. This is effective to realize low-noise and low-voltage Ge APDs [3]. In this work, such Ge/SiGe heterostructure APDs are fabricated, and the multiplication gain is measured. As a result, the growth temperature in ultrahigh-vacuum chemical vapor deposition (UHV-CVD) is found to be a key parameter to enhance the multiplication gain. UHV-CVD was used to grow Ge/graded-SiGe/Ge heterostructures for free-space APDs on Si. GeH4 and Si2H6 were used as the source gases. First, 350-nm-thick undoped Ge layer was directly grown on p+-Si using low/high-temperature two-step growth. The Ge buffer layer (~50 nm) was grown at 370°C, followed by the growth at an increased temperature. As an important parameter, we compared the growth temperature, 510°C and 580°C. Then, a 24-nm-thick SiGe layer with the graded Ge composition from 70% to 100% was grown at the same temperature of 510°C or 580°C. Finally, a 50-nm-thick Ge layer was grown, and P atoms were implanted to form the n-Ge optical absorption layer. The valence band discontinuity at the bottom Si0.3Ge0.7/Ge interface is as large as 0.1 eV, assisting holes to reach the threshold energy (~1.0 eV) required for the impact ionization. As a reference, a Ge homojunction APD without the SiGe layer was also fabricated. Figure 1(b) shows typical hole-initiated multiplication gain as a function of applied voltage. Photo-generated holes can be selectively injected from the top n-Ge absorption layer to the bottom SiGe/Ge multiplication layer, using the free-space illumination of 532 nm laser light whose penetration depth in Ge as large as 20 nm. The use of SiGe/Ge multiplication layer is found to increase the multiplication gain in comparison with the Ge homojunction APD, being effective for the low-voltage APD operation. Furthermore, the Ge/SiGe APDs formed at the lower growth temperature of 510°C showed larger multiplication gain than that for 580°C. This is ascribed to the formation of abrupt SiGe/Ge interface at the lower growth temperature, as confirmed by the x-ray diffraction measurements. Excess noise characteristics will be also presented. [1] L. Virot et al., Nature Commun. 5, 4957 (2014). [2] Y. Kang et al., Nature Photon. 3, 59 (2009). [3] Y. Miyasaka et al., Jpn. J. Appl. Phys. 55, 04EH10 (2016). Figure 1

Avalanche characteristics of thin GaAs/Al 0.6Ga 0.4As heterojunction avalanche photodiodes

The mean multiplication gain and excess noise factor of thin GaAs/Al 0.6Ga 0.4As heterojunction avalanche photodiodes (HAPDs) are simulated. The ionization coefficients of electron and hole in bulk GaAs and Al 0.6Ga 0.4As are used in this model to study the role of heterojunction in reducing excess noise. The band-edge discontinuities at the conduction and valence bands are included in our model which may influence the number of carrier crossing the heterojunction and hence modifies the dead space in the HAPDs. The mean multiplication gain and excess noise factor with electron- and hole-initiated multiplication for 0.1 and 0.2 μm multiplication lengths in GaAs/Al 0.6Ga 0.4As HAPDs are shown. By considering the dead space effect, our model demonstrated a small noise mainly due to the localization of carrier ionization and the limited carrier feedback ionization at heterointerface. In our model, most of the ionizations occur in the first-initiated multiplication layer which reduces the randomness of carrier ionization and noise.

High quantum efficiency back-illuminated GaN avalanche photodiodes

Back-illuminated avalanche photodiodes (APDs) composed of heterojunctions of either p-GaN/i-GaN/n-AlGaN or p-GaN/i-GaN/n-GaN/n-AlGaN were fabricated on AlN templates. At low voltage, an external quantum efficiency of 57% at 352 nm with a bandpass response was achieved by using AlGaN in the n-layer. Dependency of gain and leakage current on mesa area for these heterojunction APDs were studied. Back-illumination via different wavelength sources was used to demonstrate the advantages of hole-initiated multiplication in GaN APDs.

Effect of doping concentration on avalanche multiplication and excess noise factor in submicron APD

A realistic full-band Monte Carlo (FBMC) model is applied to study the effect of doping concentration on multiplication gain and excess noise factor for electron- and hole-initiated multiplication in thin InP p +–i–n + diodes with a range of multiplication lengths of w = 0.1 and 0.24 μm. This model predicts a reduction in excess noise factor for both electron- and hole-initiated multiplication as the doping concentration increases. Besides dead-space effect and feedback impact ionization, the electric field profile controlled by the doping concentration significantly contributes to the fall of excess noise in submicron InP p +–i–n + diodes.

Scaling in back-illuminated GaN avalanche photodiodes

Avalanche p-i-n photodiodes of various mesa areas were fabricated on AlN templates for back illumination for enhanced performance through hole-initiated multiplication, and the effects of increased area on device performance were studied. Avalanche multiplication was observed in mesa sizes up to 14063μm2 under linear mode operation. Uniform gain and a linear increase of the dark current with area were demonstrated.

Geiger-mode operation of back-illuminated GaN avalanche photodiodes

The authors report the Geiger-mode operation of back-illuminated GaN avalanche photodiodes. The devices were fabricated on transparent AlN templates specifically for back illumination in order to enhance hole-initiated multiplication. The spectral response in Geiger-mode operation was analyzed under low photon fluxes. Single photon detection capabilities were demonstrated in devices with areas ranging from 225to14063μm2. Single photon detection efficiency of 20% and dark count rate <10kHz were achieved in the smallest devices.

Hole-initiated multiplication in back-illuminated GaN avalanche photodiodes

Avalanche p-i-n photodiodes were fabricated on AlN templates for back illumination. Structures with different intrinsic layer thicknesses were tested. A critical electric field of 2.73MV∕cm was estimated from the variation of the breakdown voltage with thickness. From the device response under back and front illumination and the consequent selective injection of holes and electrons in the junction, ionization coefficients were obtained for GaN. The hole ionization coefficient was found to be higher than the electron ionization coefficient as predicted by theory. Excess multiplication noise factors were also calculated for back and front illumination, and indicated a higher noise contribution for electron injection.

Full-band Monte Carlo simulation of thin InP p+–i–n+ diodes

A full-band Monte Carlo (FBMC) model is developed to simulate the avalanche characteristics of thin InP p+–i–n+ diode. The realistic energy band structure of InP used in this model is generated from the local empirical pseudopotential method. The electron and hole ionisation coefficients are fitted to the available measurement in the wide range of electric fields with a softer threshold than the Keldysh model. The multiplication gain and excess noise factor of electron-and hole-initiated multiplication in the thin InP p+–i–n+ diodes were simulated using FBMC model. Our FBMC results are compared to a simple random path length (RPL) model. The FBMC model predicted lower noise as compare to the results simulated from RPL model in thin InP p+–i–n+ diodes.

Fabrication and Measurement of 4H-Silicon Carbide Avalanche Photodiodes

ABSTRACTAvalanche photodiodes (APDs) that are sensitive in the ultraviolet (UV) from approximately 250 – 350 nm have been fabricated from 4H-SiC. The SiC APDs, which use hole-initiated avalanche multiplication, were produced using n-type SiC epitaxial layers grown on a p-type substrate. In order to achieve avalanche breakdown in the bulk of the material, a dry ion-based etching technique was used to form sloped sidewalls on the devices. The devices had an area of approximately 1 mm2, and they had maximum breakdown voltages of approximately 500 V. The APDs had a positive temperature coefficient for avalanche breakdown voltage and showed excellent stability for multiplication factors in excess of several hundred. Dark current, photo responsivity, and multiplication measurements from room temperature to 150°C will be presented. The dark noise performance of the APDs has also been characterized using a standard nuclear spectroscopy system consisting of a charge sensitive preamplifier, a shaping amplifier, and a multichannel analyzer. The input equivalent dark noise charge and excess noise factor for the dark current was measured over a range of shaping times, temperatures, and bias voltages. The noise performance of SiC APDs in applications such as gamma ray or x-ray spectroscopy will be highly dependent on the achievement of low bulk leakage current at the operating point. Here, an ionization coefficient ratio (k=α/β) of 0.078 was found.

Dead space effect in space-charge region of collector of AlGaAs/InGaAs p-n-p heterojunction bipolar transistors

Hole-initiated avalanche multiplication is investigated using an AlGaAs/InGaAs p-n-p heterojunction bipolar transistor (HBT). Both experimental measurements and theoretical calculation are used to determine the avalanche multiplication factor. A large departure is observed at low electric field when comparison is made between the measured data and theoretical results obtained from the standard ionization model. The comparison shows that the conventional impact ionization model, based on local electric field, substantially overestimates the hole avalanche multiplication factor Mp−1 in the AlGaAs/InGaAs p-n-p HBT, where a significant dead space effect occurs in the collector space-charge region. A simple correction model for the dead space is proposed, that allows the multiplication to be accurately predicted, even in a heavily doped structure. Based on this model, multiplication characteristics for different threshold energy of the hole are calculated. A threshold energy of 2.5 eV was determined to be suitable for describing the hole-initiated impact ionization process.

Monte Carlo simulation of high-field transport and impact ionization in AlGaAs p+in+ diodes

We have used Monte Carlo simulation methods employing both realistic band structure and a simpler analytical approximation to investigate impact ionization in bulk AlxGa1-xAs and also submicron p+in+ diodes for x40%. The calculated impact ionization rates in bulk AlxGa1-xAs compared well with previous experiments and the electron- and hole-initiated current multiplication characteristics of the p+in+ diodes were found to agree very well with our experimental results for both the analytical and the numerical models.