Transcript LAVAMAC Report LAser strobed Vacuum Assisted Microelectronic A/D Converter Katerina Ioakeimidi March 20, 2002

LAVAMAC Report
LAser strobed Vacuum Assisted
Microelectronic A/D Converter
Katerina Ioakeimidi
March 20, 2002
Talk Outline
E-Beam Digitizers
Our Approach
Improvements Over the State-of-the-Art
Photocathodes for LAVAMAC
Streak Camera Experiment
E-Beam Digitizers
E-beam digitizers with pulsed detector plates
Tektronix 10-100MHz, 4 bits

R. Hayes,Industrial Research and Development, May 1980, p.124
E-beam digitizers using CRTs with pulsed sweep voltage
1GHz, 4 bits with multiplexing options.

U.S. Patent# 4,005,407, U.S. Patent# 4,005,408
What’s new about LAVAMAC?
Uses MEMS technology and high rep rate modelocked
lasers to increase the analog bandwidth to more than
50GHz!!!
Also high rep rate introduces cryptography opportunities
Our Approach
Novel use of a mode-locked laser and
NEA GaAs photocathode to create subpicosecond, 5μm diameter, 100 GHz
electron pulses
Detection achieved using ultra-fast
response LT-GaAs MSM electron
detectors
Basic LAVAMAC Structure
4-bit, 100GHz, A/D Converter
2. Electron pulses are created at the photocathode
and accelerated to the anode.
3. Vin/2 analog signal is applied on the
deflector plates to alter the path of the electron
pulse.
0100
1111
0010
1100
0110
1011
4. The deflected electron bunches hit the MSM
detector code plate and are assigned a spatial bit value.
1. Laser strobe
200fs, 100GHz
800nm
Aperture plate and electron deflector plates
Anode at 0V
NEA GaAs Photocathode
at –5000V.
Metal-Semiconductor-Metal (MSM)
electron detector code plate
Improvements Over Current
Technology
LAVAMAC
4-bits @
100 GHz
Code Plate Design
Considerations
To match electrooptic
beam spot and timespread limitations,
detectors will have 10
μm square active area
and buffer region of 5
μm.
Detectors will be
assigned bit values
vertically
10 μm device
length
5 μm buffer
region
16 detectors per column
‘Master’ column
Code Plate Error Correction
A B
A B
A B
1011
1010
1001
1011
1010
1001
1011
1010
1001
1000
0111
0110
1000
0111
0110
1000
0111
0110
Optimal Case
Small Shift
Large Shift (Round Down)
Emitter Requirements
1 psec electron pulses,every 10psec, 3 micron diameter,
are created at the photocathode and accelerated to the anode.
10psec
0100
1111
0010
1100
0110
1011
Laser strobe
200fs, 100GHz
800nm
Aperture
plate and
deflector
plates
Anode at 0V
Photocathode
at –5kV
10micron diameter
beam spot size,
50 electrons per pulse
Metal-Semiconductor-Metal (MSM)
electron detector code plate ,0Volts
16 detectable levels for 4 bits
Photocathodes for LAVAMAC
Pulses condense in time
METAL: 10-15-10-14 sec from electronelectron scattering
NEA: 10-10-10-9 sec from electronoptical phonon scattering
10psec
•Energy Spread
Photocathode
•METAL: >800nm
0.5eV
•NEA: ~ 0.1eV
ђω
200fsec
•Theoretical
calculations from Spicer et
al.,SPIE,vol.2022,(1993), p.18
•Spicer W. , J. Phys. Chem. Solids, vol.2,
pp.365-370,
100 GHz Rep(1971)
rate lasers available.
•Angular Spread
•METAL: Cosine Distribution
•NEA: <12degrees semicone angle
Condense in space and
phase space (#bits)
QE (heat)
METAL: 10-5-10-4
NEA : 10-2-10-1
10psec
1psec
50e/pulse due to space charge
Spot size outside anode
Max energy spread 10meV
5μm
Time-Broadening of Electron
Pulses
Anode 0V
500Vin/2
microns
Analog signal
500fsec electron pulse
Following photoabsorption
5 microns
and generation
<E>=100meV
F of electron pulses,
5 microns
time-broadening takes place due to transit time and scattering effects.
Fcouloub
FU
couloub
200fsec light pulse
30 microns 180nm U
650fsec500fsec
800fsec
electronelectron
pulse pulse
0 Volts
-5
kV
Time spreading
also occurs due to rapid
50nm
transient of the deflecting electric field and
GaAs
0100
1111
0010
1100
0110
Longitudinal
Time
spread
velocity
due to space
spread
charge
just 1011
at
fringing effects.
outside
low
longitudinal
cathode velocity
Time spread is around 0.2 psec and it
Electric
field, pumping
surface roughness.
Equation
of motion
for far leftlevel,
particle
increases
with
increasing
sweep speed.
2=(q/m)(E
Assume
longitudinal
energy
spread E=200meV
and mean
d2x/dt
accel-Ecoul)
initial7This
energy
100meV.
effect
is electrons
not important
if the plates are
Eaccel=10
V/m
Drift
time
of
inside
The velocity
spread
is:U=E/mU0=1.87x105m/sec
2
2
positioned
very
close 1to plates
the anode,
Ecoul=1.6*(q
/4πε0plate
r12 )and electron
U 0 t thus the
Aperture
2
cathode
The
time
spread
t is: d  Udeflector
 telectron
 t  pulse 110
f sec
0 t the
broadening
only
Calculatetime
Velocity
from the sameof
equation
2
t  U 0
Cathode thickness, p-doping,cathode
Metal-Semiconductor-Metal
(MSM)
occurs
after
sampling
of the
signal.
Equation
for
Time
Spread
intensity
material,
light
pulsethe
where

is
the
acceleration
due
to
the
electric
field.
electron detector code plate
Anode at 0V
 dx=0.1micron
U(t)dt
graded
bandgap
GaAs
cathode~ 1
Total pulse
length
1
psec
psec pulse (~107cm/sec drift velocity for
E>5kV/cm).
NEA GaAs Photocathode
Laser strobe
200fs, 100GHz
800nm
at –5000V.
Spot Size Spread
L = 2mm
30 microns
2 micron spot
size optical
pulses,
200fs, 100GHz
800nm
SpotVin/2
sizeAnalog
spread
due toenergy
transverse
For
a transverse
Input
spread of 10 meV,
the spread
of the
size is:
energy spread
after
thespot
anode
Time of flight between anode and cathode:
5 microns
t  dm/ qE =25psec
U1tr
<Utr>
U2tr component:
Transverse
velocity
10 microns
Ur  - U2
2E >|=10meV
/ m  1.8 105 m/sec
|<U1
tr
tr
Diffraction-limited
3 micron
electron beam spot size
Total spot spread :
10 microns
y = Ur direction
t = 4.2mfor
Beam spread y, in the transverse
small deflection angles is:
Spot size spread due to space charge is
y=L ΔUtr/Uh=6.6 microns for L=2mm.
negligible because the spot size is much
larger than the longitudinal dimension of
500 microns
Metal-Semiconductor-Metal (MSM)
the pulse. (180nm for 100meV pulse)
electron detector code plate
Anode at 0V
NEA GaAs Photocathode
at –5000V.
0100
1111
0010
1100
0110
1011
diameter, 50 electron pulses every
10psec
Udr~107cm/sec
Glass
Substrate
Laser
pulses
~0.1eV Bandgap change
E=5kV/cm
AlxGa1-xAs
x=0.15
Cs,O layer
5 x1018cm-3 Be doping
GaAs
Vacuum
x1019cm-3
x=0.05
5
Be doping
~15nm
75nm
GaAs/AlGaAs active region
800nm available lasers(1)@100GHz
(1) IEEE,QE(1),p.539,1995
Transport time 75nm/(107cm/sec)<1psec
Longitudinal energy spread 27meV(2)(5keV)
Space charge outside photocathode
Beam spot size at cathode 3μm(3)
Transverse energy spread 25meV(2)
QE=5%(4)
Space charge inside photocathode(5)
~1psec total pulse length at the deflector
Beam spot size on detector 10μm
50 electrons/pulse min detectable requirement
Photocathode transport
analysis
Parameters that affect the transport time of electrons in submicron thin
p-doped AlxGa1-xAs layers:
1. Doping- e-h interaction
2. Thickness of layer -Overshoot (ballistic) transport for ~100nm
layers
3. Build in electric field- Grading rate
4. Injection of photocarriers
Design of photocathode
Udr~107cm/sec
for subpsec electron transport
E=5kV/cm
~0.1eVbandgap change supported by pump probe
Glass
Substrate
x=0.15
AlxGa1-xAs
5 x1018cm-3
Be doping
x=0.05
measurements of thin graded
AlGaAs layers:
E=8.8kV/cm
GaAs Cs,O
5 x1019cm-3
Be doping
~15nm
1
2
3
75nm
1.
2.
3.
5μm Al 0.65Ga 0.35As window layer
0.42μm Al 0.3Ga 0.7As – GaAs graded layer
5μm Al 0.65Ga 0.35As window layer
Transit time measured with pump probe techniques:
t=1.7psec (APL42 ,769)
1psec long electron pulses
75nm NEA
photocathode
Transport time inside photocathode:
ttr= 75nm/(107cm/sec)=750fsec
+
500μm
Pulse time spread due to
longitudinal energy spread
750fsec electron pulse
t 
ΔΕ=27meV(2) 5 microns
(3)
3μm
FCoulomb
-5 kV
20psec,
400nm
GaAs
2psec,
Space charge outside cathode
Fcoulomb for 50 electrons/3 μm diameter ,
1pseclong  ~100fsec pulse broadening
Bulk
6psec,
+
0 Volts
~900fsec electron
pulse
U 0t
 12 f sec
t  U 0
200nm
GaAs
Time resolved measurements on a psec time
scale performed at a pulsed 100keV
electron gun facility at Mainz, Germany(6).
Experimental results for 3 different samples
of >1micron, 400nm and 200nm thickness
are shown in figure 3.
In figures 4,5,6 the experimental results are
fitted by the diffusion model.
2Pastuszka
et al.,APL,71,p.2968
Baum thesis, Stanford University
6Schuler, J. et al., in the Workshop on polarized electron
3A.
3μm diameter spot size and
Space charge inside photocathode
50electrons/pulse
Je
Cs.O
?ω
JR
JS
Vacuum Level
L = 2mm
25 microns
Vin/2 Analog Input
2 micron spot
size optical
pulses,
200fs, 100GHz 5 microns
800nm
Positive
Surface States
10microns
+
+
+
0100
1111
0010
1100
0110
1011
Jr
5microns
Diffraction limited 3μm(3)
electron beam spot size
Transverse energy spread
25meV(2)
JR: recombination current in the bulk of the active region
JS: recombination current at the surface states
Jr: Hole recombination current
Je: Escape current
10 microns
Cathode,-5keV
500 microns
Anode, 0V
Metal- Semiconductor
- Metal (MSM)
electron detector code plate
Neutralization of surface
states decreases the amount
of positive charge at the
surface and raises the
vacuum level
5P.
Paez thesis, Stanford university
JR, threshold = 1012e/cm2/nsec(6)
50electrons/10μm2/psec = JR, threshold for QE=3%
2Pastuszka
et al.,APL,71,p.2968
3A. Baum thesis, Stanford University
Decrease space charge:
Increase QE
QE of 100nm GaAs active layer
Be ,1x1018cm-3
grown at Stanford by Kai Ma
% QE
3
2 .5
2
1 .5
1
S e rie s 1
0 .5
0
650
900 nm
UV treatment before activation
Structures for 1psec response
photocathodes
1. 350 microns GaP wafer >
5x10^17/cm3 p-doped
2. ~0.1 microns GaP buffer
layer >5x10^17/cm3 p-doped
3. 1.5 microns Graded bandgap
In(x)Ga(1-x)P x->(0, 0.49)
>5x10^17/cm3
p-doped
4. 1.0 microns
In(0.49)Ga(0.51)P
>5x10^17/cm3 p-doped
5. 60 nm, Graded Bandgap
Al(y)Ga(1-y)As y->(0.150.10), 5*10^18/cm^3
p-doped
6. 15 nm, GaAs ,
5*10^19/cm^3 p-doped
E=5kV/cm in active graded
area,
Udrift=107cm/sec
Transit time : <1psec
GaAs
AlyGa(1-y)As
In.49Ga.51P
InxGa(1-x)P
GaP
laser
Different spin transport paths
D± (ρe )= D0 (1±r ρe )
μ± (ρe ) =e D± (ρe )/kBT
D± (ρe ), μ± (ρe )
are the diffusivity and mobility for up and down spin electrons moving in partially
polarized electrons .
ρe =(N+(x,t)-N-(x,t))/ (N+(x,t)+N-(x,t))
is the degree of electron spin polarization
r
is the parameter to represent the strength of the spin dependence
r=0.5)
(e.g.
(PRB61,5535)
Ballistic transport not considered here, where conduction band splitting can cause
spin precession angle changes (PRL 52,2297)
This is also related to the coherent time of spin polarized electrons
inside the photocathode
Experiments
Measure:
Time length of electron pulses
# of electrons/pulse
Beam spot size (deflector off)
Charge distribution of pulses
Charge limit effects in psec scale
Analog signal
Demonstrate 1-4 bit principle of operation
of LAVAMAC
Streaked Image on Phosphor
screen
~10psec
~1psec
80MHz Sinusoid
analog signal
synchronized with
laser pulses
~10psec
200μm/psec
100psec/2cm
Different amplitudes
Different height levels
on phosphor screen
Streak Camera Experiment
Streaked Image
~10psec
~1psec
~10psec
200μm/psec
100psec/2cm
Hardware Assembly
Ti sapphire laser
Streak camera
Streak Camera
Further work/ideas
Activate thin layers of GaAs photocathodes(samples
of 20,40 60nm 1018cm-3 and 10nm 5*1019cm-3
p doped GaAs
Measure mobility with Hall effect
NEA or PEA? For transport
Charge limit
Energy distribution/Quantum well structures
Metal vs GaAs cathode high frequency response and heat
dissipation
Cs/NF3 layer Schotky barrier? Used for resonant
tunneling?
Monte Carlo sims
Further Issues
Traveling Wave deflection plates
Amplifying 50GHz bandwidth signals
Focusing
Alternative methods of measuring time
length of sub-psec electron pulses