TECHNOLOGY VLSI SYMPOSIUM


devices can realise a saturated sub- threshold swing of 190 mV/decade.


“While magnesium doping is effective at reducing the off-state leakage, it does impact electron mobility and device performance, so we are pursuing device designs that need lower or no magnesium doping,” comments Waldron. Efforts are also being directed at achieving a low defect density in the III-V layer when the process is scaled to 10 nm or 7 nm.


Superior scaling Meanwhile, researchers at UCSB are claiming to have fabricated the first III-V MOSFETs that have on-current, off-current and operating voltages comparable to or exceeding production silicon devices – while being constructed at dimensions that are relevant to the VLSI industry.


Devices produced by the team, which is led by Mark Rodwell, Arthur Gossard and Susanne Stemmer, have a 25 nm gate length, can operate at 0.5 V, and produce an on-current of 0.5 mA and an off-current of 100 nA/μm.


To set a new benchmark for III-V MOSFET performance, modifications to the conventional device architecture included a trimming of the InAs channel thickness


researchers at UCSB are claiming to have


“


fabricated the first III-V MOSFETs that have on- current, off-current and operating voltages comparable to or exceeding production silicon devices – while being constructed at dimensions that are relevant to the VLSI industry


” Meanwhile,


to just 2.5 nm. Sanghoon Lee from the team told Compound Semiconductor that the thinner channel aids the off- state current because it increases the quantised band gap of the InAs quantum well, leading to a reduction in band-to- band tunnelling, which is likely to happen near a high drain field region. Another benefit of a thinner channel is an increase in gate capacitance, which could help boost on-current.


The high-performance of these MOSFETs has been aided by the development of a high-quality gate insulator, made from the pairing of Al2


O3 and ZrO2 .


“We have used zirconium oxide instead of hafnium dioxide, because MOS capacitor analysis for zirconium oxide verses hafnium dioxide tells us that the permittivity of zirconium oxide, typically 23, is larger than that of hafnium dioxide, 19,” explains Lee. However, the total gate capacitance would not increase by much if ZrO2


more common HfO2


was replaced by the . According to Lee,


that’s because it is the semiconductor capacitance in the III-V MOSFET, rather than the oxide capacitance, that is the limiting factor for the total gate capacitance.


Due to this, Lee argues that the impressive performance of the MOSFET is essentially down to the insertion of a thin channel, plus the introduction of a vertical spacer that leads to a smoother distribution of the field within the device. By ironing out the spikes in the electric field profile, band-to-band tunnelling is prevented and leakage currents are reduced.


MOSFETs were formed from epistructures grown by MBE on semi- insulating InP. After depositing a 50 nm-thick unintentionally doped InAlAs buffer, engineers added: a 250 nm-thick, p-doped InAlAs barrier; an unintentionally doped 100 nm-thick InAlAs barrier; a 2 nm-thick InAlAs n-type, pulse-doped layer; a 5 nm-thick unintentionally doped InAlAs setback; a 3.5 nm-thick strained InAs channel; and a 2 nm-thick, unintentionally doped In0.53


Ga0.47 spacer (see Figure 6).


Figure 6. The vertical spacer and the thin InAs channel are claimed to hold the key to the high-performance of the UCSB MOSFET.


Dummy gates with lengths ranging from 12 nm to 1000 nm were formed by coating wafers with a 20 nm-thick film of the photoresist hydrogen silsesquioxane, and patterning the surface with electron- beam lithography. MOCVD added the


Copyright Compound Semiconductor Issue VI 2014 www.compoundsemiconductor.net 53


As


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160