Technical Report

Development and Simulation of UV 4H-SiC

Single Photon Avalanche Diodes

Technical Report by Vadim Pogoretskiy

Abstract: This work reports the simulation and design of the 4H-SiC single photon avalanche diodes (SPAD) for radiation detection in NUV and MUV scale of the spectrum. Absorption and impact ionization mechanisms in 4H-SiC were investigated. LabVIEW code for Poisson equation solving was developed. Two construction solutions: vertical and planar SPAD structures with breakdown voltage of 122 and 65 volt respectively were simulated. CVD epilayers for these samples were fabricated and the mask set for bevel termination and contact pads formation was developed.

Microand nanoelectronics department

St. Petersburg Electrotechnical University

St. Petersburg 2014

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1. Introduction

A single photon avalanche diode (SPAD) is a solid-state photodetector in which a photogenerated carrier can trigger an avalanche current due to the impact ionization mechanism. This device is able to detect low intensity signals (down to the single photon) and to signal the arrival times of the photons with a jitter of a few tens of picoseconds [1].

SPADs use the photon-triggered avalanche current of a reverse biased p-n junction to detect an incident radiation. SPADs are specifically designed to operate with a reverse-bias voltage well above the breakdown voltage. This kind of operation is also called Geiger mode, by analogy with the Geiger counter [1].

Silicon carbide exists in at least 70 crystalline forms. 4H-SiC, with a wide band gap of 3.28 eV, is a promising semiconductor material for high power, high temperature, and high frequency applications, owning to its high breakdown electric field, high electron saturation drift velocity, high thermal conductivity, radiation-hardness, high temperature operation [2]. For SPAD applications 4H-SiC has a high impact ionization rates asymmetry which decreases excess noise. Also it has a high visible blindness in NUV and MUV range [2].

2. Absorption in 4H-SiC

One of the most significant impacts to the absorbtion mechanisms gives the fundamental absorption which implies an optical transition of electron from the valence band to the conduction band or an excitonic state. Due to the fact that 4H-SiC has an indirect band structure one can observe indirect transitions, in which phonons have to provide an additional wave vector

Hamiltonian of the electron-photon interaction:

For direct transitions (first order of the perturbation theory):

For indirect transitions (second order of the perturbation theory):

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Given Fig. 1, absorption dependence can be fitted as:

Fig. 1. Absorption coefficient for 4H-SiC [3].

Temperature dependence of absorption coefficient can be expressed as [4]:

There are two factors required for triggering of the avalanche in semiconductor. The first is the presence of the initial carrier which can trigger an avalanche current. The second is the

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5. Simulation of 4H-SiC SPADs

In this work two unique constructions with bevelled terminations: vertical and planar were developed and simulated. Bevel termination is used to eliminate the electric field crowding at the device edge in order to avoid surface breakdown. The cross-sectional views and the doping profiles of the constructions in Fig. 2 are presented.

Fig. 2. Cross-sectional views of the constructions (left) and doping profiles (right). Vertical (top) and planar (bottom). bevel angle .

quantum efficiency were calculated. The results of the computations in Fig. 3-7 and Tables 1-2 are presented.

Fig. 3. Doping profile of the structures. Vertical (left) and planar (right).

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Fig. 4. Electric field of the structures . Vertical (left) and planar (right).

Fig. 5. Potential through the structures . Vertical (left) and planar (right).

Fig. 6. Impact ionization coefficients for the structures. Vertical (left) and planar (right).

Fig. 7. Multiplication coefficients of the structures . Vertical (left) and planar (right).

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Table 1. Input and calculated output parameters of the program for the vertical structure.

Fig. 8. Dependence of quantum efficiency (a) and penetration depth (b) on wavelength for both vertical and planar structures.

6. Optimization opportunities

The develoded code is able to perform optimization of structures within chosen parameters and target function.

Fig. 9. The example of optimization computation of one of the constructions . Reverse voltage as a target function and light doped density and width as parameters.

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7. Fabrication of 4H-SiC SPADs

The samples with grown CVD epilayers were fabricated and the mask set was developed. In order to perform diode structure the bevelled mesa termination and contact pads have to be made. Many edge termination technologies, including planar field, guard rings, junction termination extension, and positive or negative bevel edge termination, have been developed for SiC high power devices. People often classify them into two types, planar edge termination and mesa edge termination. Planar edge termination is often formed by ion implantation, which is the only selective area doping technology practically for SiC. The ion implantation energy of the dopants for SiC is much higher than that for Si for the same doping depth and the activation

fabrication. Thereby, in our case it was decided to use mesa structure with negative bevel (the area of the junction increases when proceeding from the highly doped side towards the lightly doped side).

For bevel formation we have to use 4 photolithography processes. The first is used in order to prevent the plasma etching damage to the mesa top surface; a 300 nm metal is deposited to define the top mesa pattern. Then, a thick photoresist is spun on the sample and the second photolithography is performed. The thickness of the photoresist is about 7 µm. The photoresist is baked in a hot oven. The angle of the photoresist bevel can be controlled by the baking time and temperature. An ICP etching is performed using the bevelled photoresist patterns as etching mask and the bevel on the PR is transfer to the 4H-SiC. Since the etching rate of SiC is much slower than that of the condensed photoresist in the ICP etching, a SiC mesa with a shallow bevel can be achieved. The process of bevel formation is reflected in Fig. 10.

After the third photolithography the Al layer is removed. Before the device passivation, a standard RCA cleaning is carried out to remove the ICP byproducts, native oxide, etc. After that, passivation wet oxide is grown and finally the thermal oxide is covered by PECVD SiO2 and Si3N4. The fourth photolithography is performed in order to deposit contact pads.

Fig. 10. The process of the bevel formation.

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