InAlGaAs layers for high electron mobility transistors

Text 2B

Read text 2B and answer the questions after the text

Memorize the following basic vocabulary and terminology to text 2B

1. heterojunction – неоднородный переход в полупроводниковом приборе, гетероструктурный переход;

2. intrinsically limited by optical photon scattering – по своей природе ограничен оптическим рассеянием фотонов;

3. electron density – плотность электронов;

4. enhance channel conductivity – повысить проводимость в канале;

5. conduction band – смещение зоны проводимости;

6. metallographic vapor phase epitaxy – металлографическая эпитаксия из паровой фазы

7. electron sheet density – пленочная плотность электронов;

8. subband occupancy – заселенность в подзоне;

9. electro charge distribution – распределение заряда электронов;

10. quantum wells (QW) – квантовые ямы;

11. intersubband scattering – внутреннее рассевание в подзоне;

12. light-induced destruction – разрушение вызванное светом

13. charge correlation – зарядовая корреляция (взаимосвязь);

Mobility of carries in quasi two-dimensional electron gas (2DEG) formed at a semiconductor heterojunction is intrinsically limited by optical phonon scattering at room temperature. For high performance devices, an increase in electron density leads to enhanced channel conductivity. The existence of DX centers in Al _x Ga_{1– x} As (x >0.22) restricts the use of a high Al mole fraction in Al _x Ga_{1– x} As /GaAs heterostructures. The small conduction band offset in such structures limits the electron density below 10¹² cm^–2. Owing to larger conduction band offset the pseudomorphic GaAs/In _x Ga_{1– x} As/Al _y Ga_{1– y} As quantum wells (QWs) have attracted much interest in high-speed devices. The best samples of those structures with very high 2DEG mobilities are usually grown by molecular beam epitaxy (MBE). Recent progress in metalorganic vapor phase epitaxy (MOVPE) has led to growth of high quality heterostructures. Additionally, δ-doping increases the electron density and mobility of 2DEG in the QW, and consequently improves device performance.

GaAs/In_0.2Ga_0.8As/Al_0.2Ga_0.8As QW grown by MOVPE at 630 °C and consisted nominally of 600 nm GaAs buffer layer, followed by 90 nm of (5 nm Al_0.2Ga_0.8As/5 nm GaAs) superlattice, 310 nm of 205 nm GaAs top barrier. The Si δ-doping (n_D =2.3_*10¹² cm^–2) was placed 10 nm from the QW in the back Al_0.2Ga_0.8As barrier. Similar δ-doping was applied 2 nm below the structure surface in order to saturate the surface states.

The high electron mobility 2DEG was formed in the investigated structure with no significant parallel conduction. Nonmonotonic dependence of both transport and quantum mobilities on electron sheet density was observed with the maximum related to the onset of the upper subband occupancy. An apparent increase of the quantum mobility was explained by the ionization of deep centers in the top barrier which changed the QW from asymmetric to the more squarelike. The resulting shift of the electron charge distribution away from the δ-layer leads to an increase of electron transport mobility in the QW. An explanation of the quantum mobility decrease after the upper subband occupation is less straightforward. The intersubband scattering presumably present in this density region has the opposite effect on the quantum mobility than the more effective remote charge screening. Moreover a light-induced destruction of charge correlations may be important. (2090)

Answer the following questions:

1) Why is the mobility of carriers in quasi two-dimensional electron gas intrinsically limited? 2) Why did pseudomorphic quantum well attract much interest in high-speed devices? 3) What is the technique to obtain the structures with very high 2DEG mobilities? 4) How was an apparent increase of the quantum mobility explained? 5) What leads to the increase of electron transport mobility in the quantum well?

Task 1. Use your background knowledge and explain the need for high electron mobility transistors. How was it achieved?

Task 2. Explain the structure of GaAs/In_0.2Ga_0.8As/Al_0.2Ga_0.8As quantum well and draw a stepwise scheme of its growth by metalogranic vapor phase epitaxy.

Task 3. Look through the second part of the text carefully and explain the characteristics of the high quality heterostructure.