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InN layers for high electron mobility transistors

Text 2C

Read text 2C and answer the questions after the text

Memorize the following basic vocabulary and terminology to text 2C

1. inherent property – внутреннее свойство;

2. increase luminescence efficiency considerably – значительно увеличить выход люминесценции;

3. saturation velocity – скорость насыщения;

4. wurtzite[6] – вюртцит;

5. Monte Carlo method[7] – метод Монте Карло (совокупность математических методов, позволяющих найти желаемое решение путем статистических испытаний, выполняемых, как правило, при помощи ЭВМ)

6. drift velocity – скорость смещения, дрейфа;

7. superior characteristics – более высокие характеристики;

8. overshoot velocity – скорость проскакивания;

9. field effect transistor – транзистор, управляемый полем;

10. cutoff frequency – предельная частота;

11. high-speed high-performance heterojunction – высокоскоростной высокоэффективный гетеропереход;

12. pulsed laser deposition technique – метод осаждения импульсным лазером;

13. impurity – примесь;

14. Hall mobility – холловская подвижность, подвижность зарядов;

15. molar ratio – молярная концентрация; мольное отношение;

16. enhanced decomposition – повышенное (усиленный) разложение, распад;

Developments in the field of III-nitride (InN, GaN, and AlN) semiconductors have been spectacular due to their highly attractive inherent properties. During the last few years the interest in the InN has been remarkable. Recent results indicate that the InN films almost meet the requirements for application to practical devices. The development in blue/ultraviolet (UV) light emitting diodes (LEDs) and laser diodes (LDs), and also high-frequency transistors operating at high powers and temperatures has proved the benefits of the nitride materials system. Indium nitride (InN) is an important III-nitride semiconductor with many potential applications. The use of InN and its alloys with GaN and AlN makes it possible to extend the emission of nitride-based LEDs from ultraviolet to near infrared region. For example, along with GaN, the InN ternary alloy, InGaN, has found application in a variety of heterostructure based optoelectronic devices, such as LEDs and lasers. The InGaN quantum wells are indispensable for light emitting devices because incorporation of small concentrations of In in the active GaN layer increases luminescence efficiency considerably.

InN was predicted to have lowest effective mass for electrons in all the III–nitride semiconductors, which leads to high mobility and high saturation velocity. The theoretical maximum mobility calculated in InN and GaN at 300 K are about 4400 and 1000 cm2/(V*s), respectively, while at 77 K the limits are beyond 30 000 and 6000 cm2/(V*s), respectively. The electron transport in wurtzite InN was studied using an ensemble Monte Carlo method. It was found that InN exhibits an extremely high peak drift velocity at room temperature. The saturation velocity is much larger than that of gallium arsenide (GaAs) and gallium nitride (GaN).

InN achieves the highest steady-state peak drift velocity: 4.2*107 cm/s. This contrasts with the case of GaN, 2.9*107 cm/s, AlN, 1.7*107 cm/s, and of GaAs, 1.6 *107 cm/s. It was concluded that the transport characteristics of InN are superior to those of GaN and GaAs, over a wide range of temperature from 150 to 500 K and a doping concentration up to 1019 cm–3. The transport characteristics were shown to be relatively insensitive to variations in temperature and doping concentration, unlike GaAs. This suggests that there may be distinct advantages offered by using InN in high frequency centimeter and millimeter wave devices.

The transient electron transport, which is expected to be the dominant transport mechanism in submicron-scale devices, was also studied in InN. It is found that an InN exhibit the highest peak overshoot velocity and that this velocity overshoot lasts over the longest distance when compared with GaN and AlN. It was predicted that InN-based field-effect transistors (FETs) have an extremely high speed with a cutoff frequency of over 1 THz for 0.1 mm gates. Thus, InN is a highly potential material for the fabrication of high-speed high-performance heterojunction FETs.

As-grown InN is always n-type with a very high background carrier concentration-an observation similar to GaN before better doping control of that material was achieved. However, there is only one report of p-type InN grown by the pulsed laser deposition (PLD) technique, which to date has never been reported in as-grown InN film or even intentionally doped InN film. There has been much speculation as to what species is responsible for high background donor concentration in the grown InN. Theoretical calculation as well as experimental result gives conflicting views and opinions regarding the major reason responsible for such high n-type conductivity. The potential candidates for such high background donors are native defects, such as N vacancy, nitrogen antisite, and impurities, such as ON, SiIn, and possibly interstitial H. According to the oldest and common view, the nitrogen vacancy is the most probable reason for n-type conductivity of InN.

Selection of GaN for the underlying layer and increased InN film thickness significantly improve the Hall mobility. A Hall mobility of about 700 cm2/(V*s) was obtained in the InN film grown on GaN even at an electron concentration of 5*1019 cm–3. A high NH3/trimetilindium (TMI) molar ratio and enhanced NH3 decomposition (by growth temperature, atmospheric pressure growth, reduced flow velocity, etc.) significantly improved the electrical properties of MOVPE grown InN film. As a result, a carrier concentration in the order of 1018 cm–3 was obtained and the highest electron mobility obtained was 730 cm2/(Vs). These are the best electrical properties ever achieved in the MOVPE grown InN film. (3989)

Answer the following questions:

1) Why have the developments of III-nitride semiconductors been spectacular? 2) What do recent development results indicate? 3) What proved the benefits of the nitride material system? 4) What is the application of the III-nitride semiconductor? 5) Why are the InGaN quantum wells indispensable for light emitting devices? 6) What was found by the ensemble Monte Carlo method? 7) What do the highest steady-state peak drift velocity and relatively insensitive transport characteristics suggest? 8) What was found while studying InN transient electron transport mechanism? 9) Why do the experimental results give conflicting views and opinions? 10) Due to what factors was the highest electron mobility obtained?


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